A loop invariant is type of assertion that remains true from one repetition of the loop to the next. Usually, the invariant describes some non-trivial relationship between the variables changed by the loop. For example, if a program has a loop which adds one to inc and subtracts one from dec then the sum of inc and dec will be a constant. Loop invariants are a common type of invariant.

a boolean expression that is true each time the loop guard is evaluated

a condition that should be applicable at the end of each iteration of the loop

a property or condition that does not change during the execution of a loop