Clear

Clear[s1,s2,…] clears values and definitions for the symbols si. 

Clear[patt1,patt2,…] clears values and definitions for all symbols whose names textually match any of the arbitrary string patterns patti.

Clear[{spec1,spec2,…}] clears values and definitions for any symbols that are equal to or whose names match any of the speci.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.