A beat is defined by its turn, not its length. Two lines of dialogue in which somebody concedes something is a beat. Two minutes of beautifully shot walking in which nothing shifts is not a beat, it is footage. The test is whether you could state what changed in a short sentence.
Beats are the resolution at which structure can actually be fixed. Notes at the level of the whole piece ("it drags") are unactionable; notes at the level of a beat ("nothing changes between the sixth and the eighth") point at the two things to cut. Every useful editorial conversation eventually descends to this level.
In performance, the same word means a pause — a held moment before a line. The two senses are related: the pause exists to let the change land.
What is a beat in a script?
The smallest unit of dramatic change: one action, exchange or realisation after which the situation is measurably different. If you cannot say what changed, it is not a beat.
What is the difference between a beat and a scene?
A scene is a continuous unit of place and time and usually contains several beats. A beat is the individual turn inside it. A scene with only one beat is generally too long for what it does.