cred·i·ble 
(krĕdə-bəl)
(krĕdə-bəl)adj.
1. Capable of being believed; believable or plausible: a credible witness; a credible explanation. See Synonyms at plausible.
2. Considered capable of achieving a goal: The party must nominate a credible candidate for governor.
3. Being of sufficient military capability to deter an attack or carry out an operation successfully: credible military force.
[Middle English, from Latin crēdibilis, from crēdere, to believe; see kerd- in the Appendix of Indo-European roots.]
credi·ble·ness n.
credi·bly adv.
Usage Note: Credible is widely but incorrectly used where credulous would be appropriate. Credulous means "believing too readily" or "gullible," as in He was credulous (not credible) enough to believe the manufacturer's claims.