X Definition (Illustrated Mathematics Dictionary) - Math Is Fun

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The letter "x" is often used in algebra to mean a value that is not yet known.

It is called a "variable" or sometimes an "unknown".

In x + 2 = 7, x is a variable, but we can work out its value if we try!

A variable doesn't have to be "x", it could be "y", "w" or any letter, name or symbol.