As floodnik said, a so called instakill crit is a critical hit which kills the enemy instantly, no matter how high his HP is.
The damage of the weapon is raised into heaven, and even a 9mm Mauser or a BB Gun will kill strongest enemies with
a devastating hit.
With a high critical chance and the "better critical" perk you can land instakills, not every crit will be an istakill
or course, but it will happen pretty randomly.