Clear

Clear[symb1symb2, ...]
clears all values of the given symbols. The arguments can also be given as strings containing symbol names.

ClearAll may not be called for Protected symbols.

The values and rules associated with built-in symbols will not get lost when applying Clear (after unprotecting them):

Clear does not remove attributes, messages, options, and default values associated with the symbols. Use ClearAll to do so.