Definition from the suttas.

“And what, bhikkhus, is right speech? Abstinence from false speech, abstinence from divisive speech, abstinence from harsh speech, abstinence from idle chatter: this is called right speech.” [SN 45.8]

Criteria to identify right/wrong speech

“Monks, a statement endowed with five factors is well-spoken, not ill-spoken. It is blameless & unfaulted by knowledgeable people. Which five? It is spoken at the right time. It is spoken in truth. It is spoken affectionately. It is spoken beneficially. It is spoken with a mind of good-will.” [AN 5.198]

Right Speech as related to other items in Noble Eightfold Path:

“Therein, bhikkhus, right view comes first. And how does right view come first? One understands wrong speech as wrong speech and right speech as right speech: this is one’s right view. […] One makes an effort to abandon wrong speech and to enter upon right speech: this is one’s right effort. Mindfully one abandons wrong speech, mindfully one enters upon and abides in right speech: this is one’s right mindfulness. Thus these three states run and circle around right speech, that is, right view, right effort, and right mindfulness.” [MN 117]