The Greek word is αληθεια (aletheia), the noun for truth. The other Greek word sometimes translated "truth" is the adverb form (translated "of a truth" or "truly").
Aletheia is used once by Matthew, three times in Mark, and twice by Luke (the adverb forms are 1,0,3 respectively).
John uses it twenty-five times in twenty-two verses! (He uses the adverb form twice.)
It is clear this is a word John likes. In his short epistles:
- 2 John (13 verses total) - 5 times in 4 verses
- 3 John (14 verses total) - 6 times in 5 verses