deserter

(dē̍*zẽrt"ẽr), n. One who forsakes a duty, a cause or a party, a friend, or any one to whom he owes service; especially, a soldier or a seaman who abandons the service without leave; one guilty of desertion.

noun

A person who has defected: apostate, defector, recreant, renegade, runagate, tergiversator, turncoat. Informal rat. See approach/retreat.

dǝˈzǝrdǝr dēˈzǝrdǝr n. a member of the armed forces who deserts: deserters from the army.

See the Introduction, Abbreviations and Pronunciation for further details.

The noun has 2 meanings:

Meaning #1: a disloyal person who betrays or deserts his cause or religion or political party or friend etc.
  Synonyms: apostate, renegade, recreant, ratter

Meaning #2: a person who abandons their duty (as on a military post)
  Synonym: defector