The squish is the distance from the top of the piston at TDC to the top of the cylinder...Less squish, more compression..
All cylinders have a dome at the top, sometimes a hemi, sometimes not..There is a narrow band, usually about 1/4 to 3/8 inch, around the top of the cylinder where the dome starts..This band is where the squish is measured...
There were a few engines that had a flat combustion chamber, with the top of the piston machined out for the combustion chamber..Some diesels, I think...
Squish is measured by putting a piece of solder into the spark plug hole and squishing it flat by rotating the crank...