A hobby is an activity that you do in your spare time for fun. Your hobby may be putting together model train sets and driving your mom crazy from all the noise.
In the 13th century the word hobby referred to a small horse or a pony. It later came to describe a toy horse — a hobbyhorse. It’s from the hobbyhorse that the word’s modern sense of “favorite pastime” evolved. A hobby is something that you do for fun — not money — and you typically do it fairly regularly. Collecting stamps, growing roses, reading — any of these can be hobbies.