Spirant
A spirant can be defined as any fricative that is not a sibilant. Consonants such as /f θ x χ/ are generally agreed to be spirants.
The spirant/sibilant contrast almost fully correlates to place of articulation, with coronal fricatives tending to be sibilants. The exceptions to this are the dental and alveolar POAs, where a contrast is possible (and the two sounds may also change to one another). Regardless, even here it usually is so that the dental spirant /θ/ is more common than the dental sibilant /s̻/, and likewise the alveolar sibilant /s/ is more common than the alveolar spirant /ɹ̝̊/.
Sound changes involving spirants
Spirants are commonly formed by lenition of corresponding stops - unlike sibilants, which are formed by assibilation (which is usually linked to palatalization). Similarly, fortition tends to produce stops from spirants (but affricates from sibilants); due to this, spirant counterparts to affricates are exceedingly rare. It is, indeed, possible to analyze a great many languages' affricates as being featurally simply sibilant stops.
Dubious cases
Though definitely not sibilants, it is not clear if lateral fricatives (eg. /ɬ/) and laryngeal consonants (eg. /h/, /ħ/ can be included as spirants. Lateral fricatives, unlike typical spirants, commonly occur as affricates; whereas glottal fricatives behave in many ways more alike sonorants.
The voiceless palatal fricative /ç/ is occasionally described as a sibilant, but articulatorily and historically should generally be considered a spirant.
