A hobby is a spare-time recreational pursuit.
A hobby horse is a wooden or wickerwork toy made to be ridden just like a real horse (which was sometimes called a "Hobby"). From this came the expression "to ride one's hobby-horse", meaning "to follow a favourite 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, creative and artistic pursuits, 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 chef may enjoy playing computer games as a hobby, while a professional game tester might enjoy cooking. 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 cigarette card or stamp collecting, but many people find it enjoyable; so it is commonly regarded as a hobby.
Amateur astronomers often make meaningful contributions to the professionals. It is not entirely uncommon for a hobbyist to be the first to discover a celestial body or event.
In the UK, 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 that is otherwise considered boring, such as train spotting or stamp collecting.
