a sale intended to dispose of all remaining stock
"The department store announced a closeout event to sell off all its winter inventory before spring arrived."
A wave which breaks all at once, as opposed to breaking progressively along its length.
"The surfer waited for the closeout where the entire line of waves crashed simultaneously rather than rolling down one by one."
In plain English: A closeout is when something is sold at a very low price to clear out all remaining stock before it stops being available.
"The store held a closeout sale to clear out its winter inventory before spring arrived."
Closeout is a deverbative noun formed directly from the verb phrase close out. It entered English to describe the act of clearing away stock or finishing a transaction at the end of a period.