Reserve driver
A reserve driver is contracted to step into a race seat at no notice if one of the team's drivers cannot start.
Also called third driver, test driver
They keep race fitness, sit through every briefing and are seat-fitted to both cars, for a job they may never be called on to do. When it happens it happens with hours of warning, sometimes less.
The role is also a holding pattern for drivers between seats and a development route for juniors, which is why a reserve is often more experienced than their lack of starts suggests.