A girl is a female human child or pubescent. Determining which specific individuals fall into the category of "girl", however, is not as simple as it may seem. For example, in many countries one is still considered a girl until reaching the legislated age of majority (for example, 18 years of age in the United States). On the other hand, to many individuals, one is no longer a child once one has completed puberty, which often happens at a much lower age than 18. Also, many countries (for example, Spain) have an Age of Consent which is different than the age of majority or other significant ages (legal drinking age, legal working age, etc.) which puts many people in the awkward position of being considered a child for some purposes and an adult for others.
For some girllovers, for example, a young-looking sixteen-year-old still appeals to them as a girl whereas an old-looking fourteen-year-old may not. Thus, when describing someone as a "girl", we must understand that, while there are vast areas of agreement, there is still a small hazier area in the middle where disagreement should be expected.
The word girl first appeared during the Middle Ages between 1250 and 1300 CE and came from the Anglo-Saxon words gerle (also spelled girle or gurle), likely cognate with the Old Low German word gör (sometimes given as kerl). The Anglo-Saxon word gerela meaning dress or clothing item also seems to have been used as a metonym in some sense.
Girl has meant any young unmarried woman since about 1530. Its first noted meaning for sweetheart is 1648. The earliest known appearance of girl-friend is in 1892 and girl next door, meant as a teenaged female or young woman with a kind of wholesome appeal, dates only to 1961.