http://www.oxforddictionaries.com/definition/english/advocate
a person who publicly supports or recommends a particular cause or policy:
a person who puts a case on someone else’s behalf:
a professional pleader in a court of justice:
Scottish and South African term for barrister.
Origin Middle English: from Old French avocat, from Latin advocatus, past participle (used as a noun) of advocare 'call (to one's aid)', from ad- 'to' + vocare 'to call'.
http://www.merriam-webster.com/dictionary/advocate
: a person who argues for or supports a cause or policy
: a person who works for a cause or group
: a person who argues for the cause of another person in a court of law
http://www.etymonline.com/index.php?term=advocate
advocate (n.) Look up advocate at Dictionary.com
mid-14c., "one whose profession is to plead cases in a court of justice," a technical term from Roman law, from Old French avocat "barrister, advocate, spokesman," from Latin advocatus "one called to aid; a pleader, advocate," noun use of past participle of advocare "to call" (as witness or advisor) from ad- "to" (see ad-) + vocare "to call," related to vocem (see voice (n.)). Also in Middle English as "one who intercedes for another," and "protector, champion, patron." Feminine forms advocatess, advocatrice were in use in 15c.
advocate (v.) Look up advocate at Dictionary.com
1640s, from advocate (n.). Related: Advocated; advocating; advocation.