terminator

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that terminates.
• noun The dividing line between the bright and shaded regions of the disk of a moon or planet that is illuminated by the sun.
• noun A sequence of nucleotides that signals the end of transcription or translation and the completion of the synthesis of a nucleic acid or protein molecule.

from The Century Dictionary.

• noun One who or that which terminates.
• noun In astronomy, the dividing-line between the illuminated and the unilluminated part of a heavenly body.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who, or that which, terminates.
• noun (Astron.) The dividing line between the illuminated and the unilluminated part of the moon.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One who finishes.
• noun biochemistry A DNA sequence which causes RNA transcription to cease and an mRNA transcript to break off.
• noun electronics An electrical device that absorbs reflection at the end of a transmission line.
• noun astronomy The line between the day side and the night side of a planet.
• noun science fiction An artificial intelligence machine, created to destroy humans (after the 1984 film The Terminator).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who exterminates (especially someone whose occupation is the extermination of troublesome rodents and insects)