# .

A congruum (plural congrua) is a positive integer h which results in a square integer when added or subtracted from some square integer. For instance, the number 96 is a congruum, since

$$10^2-96 = 4 = 2^2$$

and

$$10^2+96=196=14^2$$

Formally, we may defined congrua as those integers h for which there exist integers x, a and b such that

$$x^2+h=a^2$$

and

$$x^2-h=b^2.$$

The first few congrua (sequence A057102 in OEIS) are: 24, 96, 120, 240, 336, 384, 480, 720, 840, 960 ... .
References

Congruum at Mathworld