Early reflections
The first sparse echoes off walls, floor and ceiling – the part that tells your ear how big the room is before the actual reverb tail takes over.
For roughly 50–80 milliseconds after the direct sound, a handful of distinct bounces arrive. You rarely hear them as separate echoes, because the Haas effect makes the brain fold them into the original, but their spacing and direction give away the room's size and shape. Only after that do they thicken into the diffuse wash people usually mean by reverb.
Digital reverbs often model them separately, with a multi-tap delay line or with the image-source method Jont Allen and David Berry published in 1979. Turning the early reflections down while keeping the tail is an old mixing trick: the vocal gets space without being shoved to the back of the hall.