Definition
Compression ignition is the process by which a diesel engine ignites fuel: air is compressed until it becomes hot enough that fuel injected into it ignites spontaneously, without any spark.
A diesel compresses air alone — typically to a far higher ratio than a petrol engine — until the air temperature rises above the fuel's ignition point. Fuel injected at the right moment ignites on contact. There is no spark and no ignition timing in the petrol sense.
Why it matters when troubleshooting
If a diesel cranks but will not fire, the question is always which of three things is missing: compression, fuel, or the heat that compression produces. Worn bores and rings, a leaking valve or a cold engine without working glow plugs all reduce the heat available. See cranks but will not start.
It also explains why a diesel needs no electrical system to keep running once started — only to start and to stop. An engine that will not shut down is a genuine emergency for exactly this reason, covered in marine engine emergencies.
Where you meet it
In every discussion of compression testing, hard starting, and why diesels are built heavier than petrol engines of the same output.