A side table is, strictly speaking, a long, narrow table that is meant to be placed alongside a wall. In modern conversation, it is often confused with either an end table, which is placed at the end of a piece of furniture, such as a sofa or couch, or a nightstand aka bedside table, which is a small table placed next to the head of a bed.
The earliest known usage of the term "side table," according to the Oxford English Dictionary, dates back to 1397.