The crossword solution for the term 'HALF OF ICOSA-' is DECA, which is a 4-letter word. DECA is derived from the Greek word 'dekatos' meaning 'TENTH'. It is used to refer to a group of ten people or things. In mathematics, it is used to refer to a group of ten elements in a set. In chemistry, it is used to refer to a group of ten atoms in a molecule. In geometry, it is used to refer to a polygon with ten sides and ten angles. Therefore, the answer to the crossword clue 'HALF OF ICOSA-' is DECA, which is a 4-letter word derived from the Greek word 'dekatos' meaning 'TENTH'.