What is "blow-by"

blowby \blow"by`\, blow-by \blow"-by`\(bl[=o]"b[imac]`), n. the leakage of gases from the combustion cylinder of an internal combustion engine between the piston and cylinder wall into the crankcase.

n. (context automotive English) in an http://en.wikipedia.org/wiki/Internal%20combustion%20engine, any burnt or unburnt matter escaping from the cylinder, past the (worn) rings on the piston, and into the crankcase.