The term 'HALF OF DECA-' can be solved by the word 'PENTA', which is a five-letter word. The prefix 'penta-' is derived from the Greek word for 'five' and is used to describe a group of five. In mathematics, pentagons are five-sided shapes and pentagrams are five-pointed stars. In chemistry, pentane is a hydrocarbon with five carbon atoms. In biology, pentadactyl refers to an organism having five digits on each limb.