Maxima help: clear variable



I've been trying to find this in a Maxima manual, but that program has 
the worst possible name for trying to search for information (please, 
please change the name to something that doesn't return results for 
Nissan vehicles and MATLAB, Octave, and Mathematica sites talking about 
local and global maxima).

After setting a variable to something, e.g.
a:2;
how do you clear that value? E.g., I would expect
clear(a);
a;
to return "a".
Currently, I have to close and reopen the program and rerun every 
command any time I need to do this.