Answer :
It depends on what the reaction is. Usually H2O is neutral. If in the reaction, it accepts proton(H+), it is base. If it donors the proton(H+), it is acid. So it based on what reaction it is.
Answer: both an acid and base
Explanation: By Bronsted-Lowery acid base theory: proton Donor is acid and proton Acceptor is base.
Here H2O by donating the proton acts as acid.
H2O ------> H^+ + OH^-
And by accepting the proton acts as base
H2O + H+ --------> H3O+
