What is squish in diesel engines?

Squish refers to the effect in a diesel/compression ignition engine when the piston approaches top dead centre.

In a direct injection system; the system is designed to make the air swirl in the combustion chamber. As the piston reaches tdc the air is reaches a high temperature, high enough to ingnite the diesel being injected into the cylinder. Squish is used to describe this final - rapid acceleration of the air caused by compression to acheive the ignition temperature.

In an in-direct injection system; the diesel is already in the air mixture as it enters the cylinder. The same process happens as per the direct injection system; ie the mixture is is compressed which raises the temperature (boyles law) to the point where ignition occurs.

Simply: Squish is where the air/fuel mixture is compressed in the combustion chamber until the corresponding temperature rise ignites the fuel.