The traditional name for the opening time trial stage in the Tour de France is a prologue.
A prologue is a very short individual time trial held before the numbered stages begin. Riders start separately and race against the clock, with the fastest time determining the opening general-classification leader. Because the course is short, often only a few kilometres, the time gaps are usually measured in seconds rather than minutes.
The format became especially associated with the Tour during the late 20th century. A prologue could take place in a city centre and was designed to create an immediate spectacle before the race moved into its longer road stages. It was not a team event: each cyclist completed the course alone.
The term is sometimes used loosely for any opening time trial, but modern Tours have not always included a prologue. Organisers may instead begin with a conventional Stage 1, an individual time trial, or another road stage. That is why “prologue” is the canonical historical answer, while “individual time trial” describes the format rather than the traditional stage name.