A geometric constraint is a limitation placed on an object, which can have two dimensions or more, because there are zero degrees of freedom. An object that is fully constrained cannot be geometrically altered; in other words, its angles and side lengths and positions cannot be changed unless the specification of one of the existing dimensions is altered first.
Geometric constraints are imposed by designating a side length or angle to be a certain value. For example, if two angles of a triangle are specified as 45 and 90 degrees, the third angle is constrained to being 45 degrees. In order to change that third angle, one of the first two angles would have to be changed first.