unknown

An unknown is the missing number a problem asks you to find. Think of a balance scale with a covered weight on one side: you know the scale is level, you know what is on the other side, and your job is to figure out the hidden weight. In algebra that hidden weight is written as a letter, often x.

An unknown is a particular kind of variable — one for which a specific value (or a small set of values) makes a given equation true. Solving the equation means using legal moves to peel away everything else until the unknown stands alone, revealing the number it was hiding all along.

Be careful: not every equation has exactly one unknown value. Some have many (x^2 = 9 has both x = 3 and x = −3), some have none in the real numbers (x^2 = −1), and some are true for every x (an identity). “Finding the unknown” really means finding all the values that make the statement true, which may be one, several, none, or infinitely many.