War dialingcomputer would repeatedly dial a number (usually to a crowded modem pool) in an attempt to gain access immediately after another user had hung up.
The term (and the technique) pre-date the movie WarGames by several years. However, the popularity of the film among computer enthusiasts led to the term being commonly used for what is more precisely known as demon dialing, which figures prominently in the movie. The expansion of accessible ISP connectivity since that time more or less rendered the practice obsolete, and today "war dialing" much more frequently refers to demon dialing.
An identical technique was sometimes used to get the first call for prizes in radio "call-in" shows, thus leading to the adoption of random "fifth caller," "seventeenth caller" etc. by radio stations to circumvent this practice.