An equivalent expression using the hydronium ion could be written. Sometimes called the "water constant", this number tells us that, among other things, the concentrations of hydrogen (hydronium) ions and hydroxide ions in pure water are equal (1.0 x 10^-7 M). This is what is meant by "neutral" water. The following conditions summarize the possibilities:

in pure water [H⁺] = [OH^-]
in acidic solution [H⁺] > [OH^-]
in basic solution [H⁺] < [OH^-]

In aqueous systems this equilibrium dominates acid/base behavior. To understand why, we need only consider a few situations in light of LeChātelier's Principle.