what does "mantra" mean and what does "brahmana" mean? Has it been defined anywhere?
The Purva Mimamsa Sutras of Jaimini, commented upon by Shabara, have defined the words "mantra" and "brahmana" as follows:
Adhyaya 2, Pada 1, Adhikarana 7: "Definition of 'Mantra'"
Sutra 32 - The name 'mantra' is applied to those texts that are expressive of the said (assertion, of things connected with the prescribed acts).
However, this is not a hard rule as Shabara says:
This definition is only illustrative, as there are certain texts which are not expressive of any assertion and are yet called 'mantra'.
Adhyaya 2, Pada 1, Adhikarana 8: "What is a Brahmana?"
Sutra 33 - To the rest (of the Veda) the name 'Brahmana' (is applied).
What does Jaimini mean by "rest of the Veda"? He means the Brahmanas proper and the Aranyakas, which include the Upanishads, since they form the ending portion of the Brahmanas. This is known because Shabara cites verses from the Brihadaranyaka Upanishad and Taittiriya Aranyaka in his bhashya:
'Ugro ha bhujyam, etc.' (Taittiriya Aranyaka 1.10.2)
So it is established that the kalpa sutras along with the purva mimamsa sutras actually claim that the samhitas, brahmanas, aranyakas, and upanishads all together form the Vedas.