"Square." It's a term you hear often in construction, and it simply means to have two adjacent surfaces that are at an exact 90-degree angle to one another. Sounds simple, but having something square is crucial for everything from wall framing and door installation to cabinets and tile.
Checking that something is square is done with a tool called, not surprisingly, a square. There are all types of squares available, some with a dedicated purpose and some that will do more than one task around the home or shop. So when shopping for a new square, it pays to have some understanding of the function of some of the more common types.