Strong's #226: aletheuo (pronounced al-ayth-yoo'-o)
from 227; to be true (in doctrine and profession):--speak (tell) the truth.
Thayer's Greek Lexicon:
1) to speak or tell the truth
1a) to teach the truth
1b) to profess the truth
Part of Speech: verb
Relation: from G227
Citing in TDNT: 1:251, 37
This word is used 2 times:
Galatians 4:16: "become your enemy, because I tell you the truth?"
Ephesians 4:15: "But speaking the truth in love, may grow up into him in all things,"