Is it ensure, insure, or assure?
1610s, from Late Latin theoreticus "of or pertaining to theory," from Greek theoretikos "contemplative, pertaining to theory" (by Aristotle contrasted to praktikos), from theoretos "that may be seen or considered," from theorein "to consider, look at" (see theory). Related: Theoretically.
theoretical the·o·ret·i·cal (thē'ə-rět'ĭ-kəl)
Of, relating to, or based on theory.
Restricted to theory; not practical.