Definition
A two-terminal semiconductor device that conducts current in either direction once the voltage across it exceeds a specific breakover value. Below that voltage it blocks current; at or above it, the diac switches on and conducts. Diacs are commonly used to trigger triacs in alternating-current power-control circuits.
Plain English
A small electronic part that stays off until the voltage pushing on it reaches a set level, then suddenly turns on and lets current pass. It works the same way no matter which direction the current is flowing, which makes it useful in AC circuits.
Context Anchor
Seen in aircraft electrical and electronic circuit descriptions, especially where a small device is used to trigger or control alternating-current switching.
Derivation
The name is built from 'diode' (a two-terminal electronic device) plus 'AC' (alternating current). So 'diac' literally means an AC diode -- a diode-like device designed to work with alternating current flowing in both directions.
Why Pilots Care
Pilots usually do not operate a diac directly, but the term may appear in maintenance or systems material for lighting, control, or switching circuits. Knowing it prevents confusing a diac with a one-way diode.
Analogy
Think of a pressure-relief valve that stays shut until the pressure gets high enough, then suddenly opens and lets flow through -- and works the same whether flow is going one way or the other.
Intuition Check
Do not assume a diac is just a regular diode. A regular diode normally conducts one way; a diac can conduct either way after its trigger voltage is reached.
Example Sentence 1
The avionics technician traced the dimmer fault to a failed diac in the trigger circuit.
Example Sentence 2
During the preflight check of the electrical system, the mechanic verified that the diac triggered the triac at the correct voltage threshold.