σύμφονα δὲ τὰ λοιπὰ ἑπτακαίδεκα· β γ δ ζ θ κ λ μ ν ξ π ρ σ τ φ χ ψ. σύμφοναι δὲ [λέγονται], ὅτι αὐτὰ μὲν καθ’ ἑαυτὰ φωνὴν οὐκ ἔχει, συντασσόμενα δὲ μετὰ τῶν φωνηέντων φωνὴν ἀποτελεῖ. (Dionysius Thrax, Art of Grammar 6.On the Sound:)

The remaining seventeen are consonants: β γ δ ζ θ κ λ μ ν ξ π ρ σ τ φ χ ψ. They are called consonants because they do not have a sound on their own, but, when arranged with vowels, they produce a sound.