Definition:a
stage worker who handles
scenery,
properties, or
lights (
Merriam-Webster)
All those folks dressed all in black that you sometimes see at the theatre. By tradition, anyone dressed completely in black can't be seen (even when you can see them), and is supposed to be ignored by the audience. And of course, the black does tend to make them disappear when they are backstage out of the limelight.
In most major cities, in the big theatres the stagehands belong to IATSE.