A "diameter hole" refers to the size of a hole from one side to the other measured straight across. It tells you how wide the hole is at the widest point. Full definition
When you're satisfied that your box is dry, glue the 5 1/2 - by - 5 1/2 - inch piece with a 2 1/2 - inch diameter hole in it 1 1/2 inches behind the front piece of the box (a piece of two - by - four can make a great spacer if available). (realtormag.realtor.org)
Start by cutting a 6» diameter hole in the tote to act as an entrance or exit. (cleanitsupply.com)
Months later, a five - foot diameter hole appeared in the surface, erupting with smoke and steam. (bookpage.com)