A sibilant is a type of fricative or affricate consonant, made by directing a jet of air through a narrow channel in the vocal tract towards the sharp edge of the teeth.
The term sibilant is often taken to be synonymous with the term strident, though this is incorrect - there is variation in usage. The term sibilant tends to have an articulatory or aerodynamic definition involving the production of aperiodic noise at an obstacle. Strident refers to the perceptual quality of intensity as determined by amplitude and frequency characteristics of the resulting sound (i.e. an auditory, or possibly acoustic, definition).
Sibilants are louder than their non-sibilant counterparts, and most of their acoustic energy occurs at higher frequencies than non-sibilant fricatives. [s] has the most acoustic strength at around 8,000 Hz, but can reach as high as 10,000 Hz. [?] has the bulk of its acoustic energy at around 4,000 Hz, but can extend up to around 8,000 Hz.
The spin-off terms shibilant, and rarely thibilant, are used to describe particular kinds of sibilant.