A sensitivity speck is a place in silver halide crystal where latent image

Latent image

A latent image on photographic film is an invisible image produced by the exposure of the film to light. When the film is developed, the area that was exposed darkens and forms a visible image...

 is preferentially formed. This is very often the site of shallow electron traps, such as crystalline defect (particularly edge dislocation

Dislocation

In materials science, a dislocation is a crystallographic defect, or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of materials...

) and silver sulfide specks created by sulfur sensitization process.

When a photon is absorbed by a silver halide crystal, a free-carrier (electron in the conduction band) is generated. This free-carrier can migrate through the crystal lattice of silver halide

Silver halide

A silver halide is one of the compounds formed between silver and one of the halogens — silver bromide , chloride , iodide , and three forms of silver fluorides. As a group, they are often referred to as the silver halides, and are often given the pseudo-chemical notation AgX...

, until captured by the shallow electron trap, where the electron is likely to reduce an interstitial silver ion to form an atomic silver. Subsequent exposure can grow the size of silver cluster through the same mechanism. This forms the latent image

Latent image

A latent image on photographic film is an invisible image produced by the exposure of the film to light. When the film is developed, the area that was exposed darkens and forms a visible image...

where the silver cluster becomes large enough to render the entire crystal developable in developer solution.