Simply, breaking-off a trout because of not giving the trout line. Alternately known as not giving the trout its "head"or not letting it run and take-out line. The break-off can occur at a leader knot or hook knot. Either way, It is a clean break (no "pig-tails"...poorly tied knot); strictly "pilot error."
There are three types:
1-grabbiing/stopping the reel's handle as it spins line off the reel
2-pinching the line to the butt section of the rod with one's line-hand
3-pinching the line to the rod handle, between the reel and control finger
There is a higher probability of the death grip to occurr with fly anglers who are relatively new to the sport and/or those who are not accustomed to fighting large trout.