The word Maya in Sanskrit breaks down to its roots 'ma' meaning "to measure or to give form" and 'ya' which translates to an article meaning "that".

Maya when applied to Brahman(the absolute transcendent reality) takes the meaning - 'that which measures or gives form to the absolute reality'.

In Quantum Theory terms, if Brahman is the Quantum wave, Maya is the collapsing of the wave function when observed or measured.

Because Brahman is the absolute truth, Maya(which we mere mortals perceive) cannot be the truth and hence termed as an illusion.

Maya is the limit of our perception which veils the transcendent truth from us.