Hot
Hot turns up in 3 fields: electrotechnology, electricians, nuclear. Here is what it means to the people in each one.
Hot in electrotechnology
Shop word for live or energised, and in North America the everyday name for a phase conductor.
Hot is the everyday word for an energised conductor or piece of plant, as in hot stick, hot-line work and working it hot. North American electricians call the ungrounded phase conductor the hot, distinct from the neutral and the ground. In control rooms a hot line or hot bus simply has voltage on it, and a hot-line tag marks a circuit with reclosing blocked while crews work it energised.
"Bus is hot from transformer 1, so don't rack anything in on that side until the switching order says so."
Hot in electricians
Ungrounded, voltage-carrying conductor of a circuit, and as an adjective anything that is live.
The hot is the ungrounded conductor, the one with voltage to ground, usually black or red on 120/240 V residential work and brown, orange or yellow on 277/480 V systems. As an adjective it means energised: a hot panel, working it hot, a hot tap. Calling a wire hot says nothing about where its voltage comes from, which is why switch legs, travelers and shared neutrals catch people out.
"Black is the hot, white's the neutral, and that red is a switched leg to the fan, check all three before you cut."
Hot in nuclear
Radioactive, contaminated or giving a high dose rate, in plant speech; the opposite of clean.
The everyday adjective for anything radiologically significant: a hot tool is contaminated, a hot job carries high dose, a hot side of a step-off pad is the contaminated side and a hot particle is a tiny intense source. Because hot also means high temperature in a plant full of hot legs and hot water, careful speakers say rad-hot or specify dose rates when there could be confusion. The opposite is clean.
"Is that wrench hot or clean? Frisk it before it goes back in the cold tool room."