'Clear' is used to refer to colourless, transparent LEGO bricks. It is given LDraw colour 47, the transparent version of white.
It is also refered to as "trans-clear" and "trans-white". Many fans prefer that usage as trans can then be a prefix to anything (trans yellow, trans white, trans light blue, trans red are all common colors...) while clear can be ambiguous. Trans-white is also sometimes humourously called "trans-invisible".