The amount trimmed off the top to "open up" pages in books, magazines, etc. Standard head trim is 3/16" but it can vary from job to job.
The distance between the head of a sheet of paper and the head of the cover (when finished). Minimum head trim should be 1/8", usually 1/4".
The distance between the edge of a folded signature and the final trim at the head of a book. Minimum 1/8", often 1/4 "