operator
/ OP-er-ay-tor /
Picture a railway level crossing right next to a station entrance: when the barrier is down it blocks the way onto the track, and when it lifts, traffic flows. In a bacterial gene switch, the operator is that barrier. It is a short stretch of DNA sitting at or near the promoter, and it is the docking spot for a regulatory protein that can either block the road or leave it open.
More precisely, the operator is a specific DNA sequence that a repressor protein recognizes and binds. When a repressor is sitting on the operator, it physically gets in the way of RNA polymerase, so transcription of the downstream genes is blocked. When the repressor leaves — because a signal molecule has changed its shape — the operator is clear and the polymerase can proceed. The operator itself does nothing on its own; it is purely a binding address, and its power comes from whatever protein is or isn't parked there.
Operators matter because they are the physical control points of operons and the clearest example of negative regulation: a gene that is kept off by a roadblock until something removes it. The lac and trp operons are both governed at their operators. Note that operators are mainly a bacterial feature; in our own cells the same idea — a sequence where a regulatory protein binds to gate a gene — shows up as regulatory sites for repressors and activators, but the word 'operator' is usually reserved for the bacterial setup.
In the lac operon, the operator overlaps the promoter; when the lac repressor sits on the operator it physically masks the start site, like a parked car blocking a driveway, so RNA polymerase cannot get going.
Repressor on the operator = a car blocking the driveway.
An operator is a piece of DNA, not a protein. The protein is the repressor that binds it — they are easy to mix up because they are always discussed together.