Spill is light that reaches an area where it is not wanted for the shot. It may come directly from a fixture, escape around a modifier or return from surrounding surfaces. Whether light counts as spill depends on the intended image.
Finding the source
Look at the unwanted area with each relevant fixture considered separately. A pale wall may be returning the key into the shadow side, while a softbox may also be lighting the background directly. Those paths need different solutions; dimming the whole key changes the useful light as well.
Practical example
For an interview against a dark wall, first check the key’s aim and the subject-to-wall spacing. A compatible grid may reduce off-axis light, while a flag can intercept a particular path. If a back light enters the lens during a camera move, check the complete move when positioning a lens-side flag.
Spill and beam specifications
Light outside a published beam angle does not necessarily disappear. Beam and field angles usually describe intensity thresholds, not a hard boundary. Also check the edge of diffusion frames and any gaps around a modifier. After controlling spill, recheck the subject and background together so that the correction has not introduced an unwanted cut or reflection.
Related terms
Grid · Flag · Feathering · Beam Angle · Field Angle · Negative Fill
References
ARRI: spill rings, snoots and louvers · Matthews: flags for spill and flare control