The term 'ONE LESS THAN PENTA-' can be solved by the word 'TETRA', which is a five-letter word. Tetra is derived from the Greek word for four, and it is used to refer to a group of four things. In mathematics, it is used to refer to a group of four numbers or a four-dimensional space. In chemistry, it is used to refer to a group of four atoms bonded together. In biology, it is used to refer to a group of four nucleotides in a DNA strand.