'Avesta' originates from Middle Persian (and ultimately Old Iranian/Avestan), via Middle Persian forms such as 'Avastāg' from an Avestan root often reconstructed as 'upastā' or similar, where the root carried the sense of 'text' or 'reading'.

'Avesta' changed from Middle Persian word 'Avastāg' (itself derived from Avestan material) and eventually entered modern English as 'Avesta'.

Initially it referred more generally to sacred texts or recitations (the idea of 'texts' or 'readings'); over time it became the fixed name for the specific corpus of Zoroastrian scripture now called the 'Avesta'.