  We expose the theoretical mechanisms underlying disorder-induced nematicity
in systems exhibiting strong fluctuations or ordering in the nematic channel.
Our analysis consists of a symmetry-based Ginzburg-Landau approach and
associated microscopic calculations. We show that a single featureless
point-like impurity induces nematicity locally, already above the critical
nematic transition temperature. The persistence of fourfold rotational symmetry
constrains the resulting disorder-induced nematicity to be inhomogeneous and
spatially average to zero. Going beyond the single impurity case, we discuss
the effects of finite disorder concentrations on the appearance of nematicity.
We identify the conditions that allow disorder to enhance the nematic
transition temperature, and we provide a concrete example. The presented
theoretical results can explain a large series of recent experimental
discoveries of disorder-induced nematic order in iron-based superconductors.
