The expressions hypotonic, hypertonic, and iostonic refer to comparisons between two solutions hence yu can't simply state that H2O is isotonic without comparing it to another solution/liquid/whatsoever.
A is hypotonic to B if A has less solute than B.
A is isotonic to B if A has the same amount of solute as B.
A is hypertonic to B if A has more solute than B.