According to the Matoran Dictionnary :
Kanohi Isima, n.cmpd. “Mask of Possibilities; allows the wearer to subtly alter fields of probability”
isi, n. “possibility”
ma(t), stm. “mastery, control”
The element isi combines straightforwardly with the element ma(t), yielding the compound isi-ma(t) “control of possibility”, modern form isima.