clear - kills variables
This command kills variables which are not protected. It removes the named variables from the environment. By itself clear kills all the variables except the variables protected by predef . Thus the two commands predef(0) and clear remove all the variables.
Normally, protected variables are standard libraries and variables with the percent prefix.
Note the particular syntax clear a and not clear(a) . Note also that a=[] does not kill a but sets a to an empty matrix.