The Compendium of Caraka 3.8.38

3.8.38

atha śabdaḥ---śabdo nāma varṇasamāmnāyaḥ; sa caturvidhaḥ---dṛṣṭārthaśca, adṛṣṭārthaśca, satyaśca, anṛtaśceti/

Now, verbal testimony: verbal testimony is a collection of sounds. It is of four kinds: referring to perceptible matters, referring to imperceptible matters, true, and false.