A nerd is a person seen as over-intellectual, obsessive, introverted, or lacking social skills. Such a person may spend inordinate amounts of time on unpopular, little-known, or non-mainstream activities, which are generally either highly technical, abstract, or relating to niche topics such as science fiction or fantasy, to the exclusion of more mainstream activities. Additionally, many so-called nerds are described as being shy, quirky, pedantic, and unattractive.

Originally derogatory, the term "nerd" was a stereotype, but as with other pejoratives, it has been reclaimed and redefined by some as a term of pride and group identity. The term may be considered a synonym for geek.

The first documented appearance of the word nerd is as the name of a creature in Dr. Seuss's book If I Ran the Zoo (1950), in which the narrator Gerald McGrew claims that he would collect "a Nerkle, a Nerd, and a Seersucker too" for his imaginary zoo. The slang meaning of the term dates to 1951. That year, Newsweek magazine reported on its popular use as a synonym for drip or square in Detroit, Michigan. By the early 1960s, usage of the term had spread throughout the United States, and even as far as Scotland. At some point, the word took on connotations of bookishness and social ineptitude.

An alternate spelling, as nurd or gnurd, also began to appear in the mid-1960s, or early 1970s. Author Philip K. Dick claimed to have coined the "nurd" spelling in 1973, but its first recorded use appeared in a 1965 student publication at Rensselaer Polytechnic Institute (RPI). Oral tradition there holds that the word is derived from knurd (drunk spelled backward), which was used to describe people who studied rather than partied. The term gnurd (spelled with the "g") was in use at the Massachusetts Institute of Technology (MIT) by the year 1965. The term "nurd" was also in use at the Massachusetts Institute of Technology as early as 1971.

According to Online Etymology Dictionary, the word is an alteration of the 1940s term "nert" (meaning "stupid or crazy person"), which is in itself an alteration of "nut" (nutcase).

The term was popularized in the 1970s by its heavy use in the sitcom Happy Days. On 28 January 1978, recurring characters The Nerds premiered on Saturday Night Live. The term was further popularized in the 1984 film Revenge of the Nerds.

Because of the nerd stereotype, many intelligent people are often thought of as nerdy. This belief can be harmful, as it can cause high school students to "switch off their lights" out of fear of being branded as a nerd, and cause otherwise appealing people to be considered nerdy simply for their intellect.

It has been argued that intellectuals are automatically nerdy because they were secretly envied, arrogant, or out of touch. However, Paul Graham stated in his essay, "Why Nerds are Unpopular", that intellect is neutral, meaning that many high school students neither admire nor deride classmates for intelligence itself. He also states that it is only the correlation that makes smart teens automatically seem nerdy, and personally defines a nerd as someone deemed not socially adept enough. Additionally, he says that the reason why many smart kids are unpopular is that they "don't have time for the activities required for popularity," since they instead prioritize intellectual, solitary pursuits, at the cost of being branded as "nerds." He also goes on to criticize suburbia and the public education system for enabling a popularity contest.