One concrete definition that is sometimes used for paraphrases requires that they be bidirectionally entailing (Androutsopoulos and Malakasiotis, 2010).
in terms of RTE, it is assumed that if P is a paraphrase of Q, then P entails Q and Q entails P. In reality, paraphrases are often more nuanced (Bhagat and Hovy, 2013), and the entries in most paraphrase resources certainly do not match this definition.