The "loid" in the title of Vocaloid is presumed to be taken from Android, with Vocal placed at the beginning, so basically "Vocal(Singing) Android" with digital vocalization.
On Crypton's website they introduce their product with this line.
" bo^karu . andoroido = VOCALOID ( bo^karoido ) desu . "
Whether this definition is accepted by other companies and Yamaha is unknown.