  We study the scaling properties of critical particle systems confined by a
potential. Using renormalization-group arguments, we show that their critical
behavior can be cast in the form of a trap-size scaling, resembling finite-size
scaling theory, with a nontrivial trap critical exponent theta, which describes
how the correlation length scales with the trap size l, i.e., $\xi\sim
l^\theta$ at the critical point. theta depends on the universality class of the
transition, the power law of the confining potential, and on the way it is
coupled to the critical modes. We present numerical results for two-dimensional
lattice gas (Ising) models with various types of harmonic traps, which support
the trap-size scaling scenario.
