A spin-wave theory of short-range order in the square-lattice Heisenberg an
tiferromagnet is formulated. With growing temperature from T=0 a gapless mo
de is shown to arise simultaneously with opening a gap in the conventional
spin-wave mode. The spectral intensity is redistributed from the latter mod
e to the former. For low temperatures the theory reproduces results of the
modified spin-wave theory by Takahashi [Phys. Rev. B 40, 2494 (1989)] and H
irsch rt al. [Phys. Rev. B 40, 5000 (1989)], and without fitting parameters
gives values of observables in good agreement with Monte Carlo results in
the temperature range 0 less than or equal to T less than or similar to 0.8
J where J is the exchange constant.