A hobby is a spare-time recreational pursuit.

In the Middle Ages, falconry was a very popular pastime (what today might be called a hobby), and of all the different birds used for it, the Eurasian Hobby was perhaps the most popular. It is said that the modern use of hobby to indicate a pastime followed from this.

An alternative explanation is that the usage grew from another recreational animal called hobby which was a type of small ambling or pacing horse. A hobby-horse was a wooden or wickerwork toy made to be ridden just like the real hobby. From this came the expression "to ride one's hobby-horse", meaning "to follow a favorite pastime", and in turn, hobby in the modern sense of recreation.

Hobbies are practiced for interest and enjoyment, rather than financial reward. Examples include collecting, making, tinkering, sports and adult education. Engaging in a hobby can lead to acquiring substantial skill, knowledge, and experience. However, personal fulfillment is the aim.

What are hobbies for some people are professions for others: a computer game tester may enjoy cooking as a hobby, while a professional chef might enjoy playing (and helping to debug) computer games. Generally speaking, the person who does something for fun, not remuneration, is called an amateur (or hobbyist), as distinct from a professional.

An important determinant of what is considered a hobby, as distinct from a profession (beyond the lack of remuneration), is probably how easy it is to make a living at the activity. Almost no one can make a living at stamp collecting, but many people find it enjoyable; so it is commonly regarded as a hobby.

In the United Kingdom, the pejorative noun anorak (similar to the Japanese "otaku", meaning a geek or enthusiast) is often applied to people who obsessively pursue a particular hobby.

While some hobbies strike many people as trivial or boring, hobbyists have found something compelling and entertaining about them. Much early scientific research was, in effect, a hobby of the wealthy; more recently, Linux began as a student's hobby. A hobby may not be as trivial as it appears at a point in time when it has relatively few followers. Thus a British conservationist recalls that when seen wearing field glasses at a London station in the 1930's he was asked if he was going to the (horse) races. The anecdote indicates that at the time an interest in wildlife was not widely perceived as a credible hobby. Practitioners of that hobby went on to become the germs of the conservation movement that flourished in Britain from 1965 onwards and became a global political movement within a generation. Conversely, the hobby of aircraft spotting probably originated as part of a serious activity designed to detect arriving waves of enemy aircraft entering English airspace during World War II. In peacetime it clearly has no such practical or social purpose.

Pursuit of a hobby may have calming or helpful therapeutic side effects. In some cases, however, (for example in collecting) the line between a hobby and an obsession can become blurred. There is more than one documented case of violence over things as simple as coin collecting.