The eyebrow is an area of thick, delicate hairs above the eye that follows the shape of the lower margin of the brow ridges of some mammals . Their main function is to prevent sweat, water, and other debris from falling down into the eye socket, but they are also important to human communication and facial expression . It is not uncommon for people to modify their eyebrows by means of hair addition, removal and makeup.
According to the 2016 United States census, roughly 3% of the population suffers from a disease called madarosis, which is a deficiency where individuals cannot grow eyebrows or eyelashes.
It is believed by anthropologists that the main function of the eyebrow is to prevent moisture, mostly salty sweat and rain , from flowing into the eye. Greek physician Herophilos , in his treatise "Anatomy," was among the first to hypothesize such a theory, proposing that the eyebrows are "adorned with hair, so that if copious perspiration came [down], it would be contained by this 'check-point' of hair placed in its way until it is wiped off, so that it could not obstruct the eyes." The typical curved shape of the eyebrow (with a slant on the side) and the direction in which eyebrow hairs are pointed, make sure that moisture has a tendency to flow sideways around the eyes, along the side of the head and along the nose. The slightly protruding brow ridges of modern humans could also be used as a defense structure against the floods of moisture. Desmond Morris, discussing the possible function of the eyebrow in human evolution , criticized this suggestion as non-effective and suggested that the primary function of the eyebrows is to signal the changing moods of their owners. Together with the brow ridges, the eyebrows also shade the eyes from sunlight. Eyebrows also prevent debris such as dandruff and other small objects from falling into the eyes, as well as providing a more sensitive sense for detecting objects being near the eye, like small insects .
Eyebrows have a facilitative function in communication, strengthening expressions, such as surprise or anger . The eyebrows can cause a "false facial expression", when a person's eyebrow shape appears to express an emotion that they are not experiencing.
Joseph Jordania suggested that the primary evolutionary function of clearly visible eyebrows was safety from predators during the vulnerable nighttime sleep, when early hominid groups started sleeping on the ground, away from the trees. Stealth predators (like big cats) as a rule stop hunting if they notice that prey animals are watching them. Cheap plastic masks, placed on the back of the head, saved many human lives in India and Bangladesh , where man-eating tigers claimed many victims in Sundarbans national park. Many predators (particularly big cats ) are very sensitive to eyespots, and all the big cats have eyespots on the back of their own ears. According to Jordania, at night, when hominid eyes were closed, the eyebrows, arched upwards, and eyelashes , arched downwards, formed clearly definable oval eyespots on a ‘sleeping’ human face, creating an illusion that the eyes were still open and watching (and therefore could deter predators from attacking sleeping hominids).
In a study published by Massachusetts Institute of Technology (MIT), where subjects were asked to identify celebrities with either their eyes or their eyebrows digitally edited out, the subjects were able to recognize the celebrity 46% of the time with their eyebrows edited out, compared to 56% of the time with their eyes edited out. The findings indicate the importance of eyebrows in providing clues to an individual's identity. 
Eyebrows are a major facial feature. Cosmetic methods have been developed to alter the look of one's eyebrows, whether the goal is to add or remove hair, change the color, or change the position of the eyebrow.
Several options exist for removing hair to achieve a thinner or smaller eyebrow, or to "correct" a unibrow , including manual and electronic tweezing, waxing, and threading. The most common method is to use tweezers to thin out and shape the eyebrow. Waxing is becoming more popular.  Lastly, there is threading eyebrows, where a cotton thread is rolled over hair to pull it out. Small scissors are sometimes used to trim the eyebrows, either with another method of hair removal or alone. All of these methods can be painful for some seconds or minutes due to the sensitivity of the area around the eye but, often, this pain decreases over time as the individual becomes used to it. In time, hair that has been plucked will stop growing back.  Some people wax or shave off their eyebrows and leave them bare, stencil or draw them in with eye liner , or tattoo them on. In Western societies, it has become more common for men to pluck part of their eyebrows.
To create a fuller look, eyebrows can be cloned in an eyebrow transplant : Individual strands of the eyebrow are mimicked to create a natural-looking eyebrow of the desired shape. The process of eyebrow transplant is quite similar to the process of hair transplant. In this process as well, follicles from an active area are transferred to the area where there are no hairs so that they can get new hairs. Follicles are mostly taken from the back of the head because it is the best harvesting site when it comes to hairs. The follicles are injected to the affected area of eyebrows so people can get eyebrows according to the desires. The famous plastic surgeon from New York City, Jeffrey Epstein says,
“Overplucked eyebrows are the most common cause of people who seek out transplants.” Unlike hair transplant where more patients are men, eyebrow transplant centers receive more women patients than men. The process is painless and normally patients start recovering after three days. The healing process after an eyebrow transplant is similar to the process of tattoo. In this process, patients come across with light bruising and crusting of the skin. People who have genetically thin eyebrows or who has overtweezed are considered ideal fir the transplant.
Within just four months of the surgery, people start getting new hair in their eyebrows and you can expect a full eyebrow area in a year. Once you get it, you can trim its hair after some couple of weeks. The brow expert who shaped the arches of Julia Roberts and Megan Fox, Tonya Crooks says that the permanent solution for all sorts of eyebrow problems is an eyebrow transplant. Crooks says,
“The hairs are usually taken from your head, which is really a different type of hair compared to the texture of your natural eyebrows.”
To get results even beyond your expectations, you need to consult a professional eyebrow transplant surgeon who contains years of experience in the field and has complete expertise as well. Though this part of your face is small in size, it plays a great role in the beauty of your face and you must not consult any unprofessional or incompetent surgeon for your .
Eyebrow brushes and shaders are used to further define the eyebrow. Other methods of achieving a fuller brow is by using brow pencils, which create an outline for the brows or mimick hairs where there are sparse areas. Brow gels are also used in creating a thicker brow; it allows for the hairs to be more textured, which gives the appearance of thicker and more fuller brows. Lastly, brow powders or even eyeshadows are used for those who want a fuller and more natural look, by placing the brow powder or eyeshadow (closest to the natural hair colour) in areas where there is less hair.
A fairly recent trend in eyebrow modification is in eyebrow tinting: Permanent dye, similar to hair colour, is used on the eyebrow, often to darken them.
An eyebrow lift, a cosmetic surgery to raise the eyebrow, usually to create a more feminine or youthful appearance, is a new phenomenon.  They can be affected during a face lift or an eye lift. More recently, doctors inject patients' eyebrows with botox or similar toxins to temporarily raise the eyebrow. 
From the 8th century, many Japanese noblewomen practiced hikimayu , shaving or plucking the eyebrow hair and painting smudge-like ones higher on the forehead. Shaving lines in eyebrows is another cosmetic alteration, more common among younger people in the 1990s and 2000s. 
A new trend is eyebrow tattoos. This is seen as a way to achieve and maintain an eyebrow shape, achieved by using a coil machine, rotary machine and linear machine. This process, also called cosmetic tattooing, microblading , or blade & shade brows, involves an eyebrow artist making small, precise cuts that mimic the look of hair  . Several celebrities have undergone the procedure, including Lorde, Lena Dunham, Adele, and Bella Thorne  .
- Anti-eyebrow , a body piercing placed below the eyebrow
- Eyebrow piercing , a body piercing done through the eyebrow
- Trichotillomania , a disorder where sufferers have an impulse to pull out body hair, including eyebrows
- Unibrow , the presence of abundant hair between the eyebrows
- Eyebrow restoration , surgical procedure to change the look of one's eyebrows