A market condition in which stocks are traded within a relatively flat range. That is, prices fluctuate around the average level, without exhibiting a discernible trend. Investors dread a sideways market because in such a market it is hard for them to make money. Market indexes attempt to make an upward or downward move but end up just about at their starting level.  Sideways trading is typically indicative of investor uncertainty. An example is the sideways market of 1966-1982 (markets tended to swing endlessly from boom to bust and then the other way around (the market was stuck in a trading range over a long period of time).

A sideways market is also known as a sideways drift or a flat market.