  A first study of critical behavior in the vicinity of the chiral phase
transition of (2+1)-flavor QCD is presented. We analyze the quark mass and
volume dependence of the chiral condensate and chiral susceptibilities in QCD
with two degenerate light quark masses and a strange quark. The strange quark
mass (m_s) is chosen close to its physical value; the two degenerate light
quark masses (m_l) are varied in a wide range 1/80 \le m_l/m_s \le 2/5, where
the smallest light quark mass value corresponds to a pseudo-scalar Goldstone
mass of about 75 MeV. All calculations are performed with staggered fermions on
lattices with temporal extent Nt=4. We show that numerical results are
consistent with O(N) scaling in the chiral limit. We find that in the region of
physical light quark mass values, m_l/m_s \simeq 1/20, the temperature and
quark mass dependence of the chiral condensate is already dominated by
universal properties of QCD that are encoded in the scaling function for the
chiral order parameter, the magnetic equation of state. We also provide
evidence for the influence of thermal fluctuations of Goldstone modes on the
chiral condensate at finite temperature. At temperatures below, but close to
the chiral phase transition at vanishing quark mass, this leads to a
characteristic dependence of the light quark chiral condensate on the square
root of the light quark mass.
