This article is from the Beer and Other Brewing FAQ, by Kurt Swanson firstname.lastname@example.org with numerous contributions by others.
Hot and cold break are terms used by homebrewers to describe the
flocculation of proteins and other materials during the boil (the
hot break) and cooling (the cold break). This material tends to
settle to the bottom of your kettle or fermenter, where it becomes
part of the "trub". Sometimes the terms "hot break" and "cold break"
will be used to refer to the activity ("I had a great cold break
when I pumped ice water through my wort chiller"), while at other
times the brewer may be referring to the actual matter ("The cold
break settled to the bottom of my carboy"); if you're worried that
you may not be understood, you can always specify whether you're
talking about the occurrence or the stuff. Usually it is understood