A spare time pursuit, practiced for interest and enjoyment, rather than as paid work. This may include activities that are in fact professions for others: for example, a computer game programmer may enjoy cooking as a hobby, while a professional chef might enjoy computer games.
While many hobbies seem, to an outsider, to be trivial and uninteresting, this is irrelevant to the hobbyist. It should be remembered that much early scientific research was, in effect, a hobby of the wealthy; and in the present day, Linux began as a student's hobby.
Another def.: Hobby: The systematic and enduring pursuit of leisure by engaging in collecting, making, tinkering, sport, adult education, and the like, doing so in a way that leads to long-term acquisition of substantial skill, knowledge, and experience. Hobbyists, unlike amateurs, lack a professional counterpart, though they may have a commercial equivalent and often have a small public who takes an interest in what they do.