I think the answer is "zero pair," but I don't think I would have gotten it without your hints! A zero pair is two manipulatives representing numbers that make zero or two numbers that make zero. Zero pairs are used in prealgebra to help students learn integer arithmetic, but most students forget about them after they no longer need to use them.
For example, an orange counter, O, can represent −1 and a green counter, O, can represent +1. Then the question, "What is −7+4" can be represented by seven orange and 4 green counters:
OOOOOOO + OOOO
A zero pair is one O and one O because together they represent moving one to the left and then moving one to the right on a number line—which is the same as not moving at all. Removing four zero pairs, or four O along with four O, results in three remaining O representing −3:
OOOOOOO + OOOO =
OOOØØØØ + ØØØØ = OOO
Another way that zero pairs are used in prealgebra is with the numbers themselves instead of with manipulatives. Then the question, "What is −7+4" is answered by forming a zero pair and removing it. The number −7 can be represented by −3−4 in order to remove the zero pair −4 and 4.
−7+4 = (−3−4)+4 = −3+(−4+4) = −3+0 = −3