Backcap
One player slipping behind enemy lines to touch or cap an objective while they focus on the fight.
Taking progress or a cap without going through the front fight — a back-take against the clock. Sombra, Tracer, Lucio and the like do it solo, forcing the enemy into the call to break off the fight and come back, which wrecks their tempo. Where a Flank is a back-take aimed at kills, a Backcap is a back-take aimed at the objective. If it lands you split the enemy and create a numbers advantage; if it fails it still turns into Stagger pressure.
When you can see the whole enemy team committed to the fight at the front. Touching the objective while escaping after a Stagger. Shaving the clock late in a KOTH round.
The one player who went behind can't get back and the fight is lost 4v5. The backcapper is the healer the fight depends on, and their absence is fatal.
Flank
Taking a secondary route to enter the enemy backline or blind spots.
Stagger
Delaying an enemy kill so they respawn out of sync with their team for the next fight.
Tempo
The initiative — who gets to choose when and on whose terms the next fight starts.