Complex sentence: Since the start of the new millennium, a new term-- "CBRNe", was introduced as a replacement term for CBRN, the "e" in this term, representing the enhanced (improvised) explosives threat.
Split into simpler sentences: Since the start of the new millennium, a new term-- "CBRNe"--was introduced as a replacement term for "CBRN". The "e" in this term represents the enhanced (improvised) explosives threat.