Fricative consonant
Fricatives are consonants produced by forcing air through a narrow channel made by placing two articulators close together. These may be the lower lip against the upper teeth, in the case of [f]; the back of the tongue against the soft palate, in the case of German [x], the final consonant of Bach; or the side of the tongue against the molars, in the case of Welsh [ɬ], appearing twice in the name Llanelli. This turbulent airflow is called frication. A particular subset of fricatives are the sibilants. When forming a sibilant, one still is forcing air through a narrow channel, but in addition, the tongue is curled lengthwise to direct the air over the edge of the teeth. English [s], [z], [ʃ], and [ʒ] are examples of this.
Two other terms are spirant and strident, but their usage is less standardized. The former can be used synonymously with "fricative", or (as in e.g. Uralic linguistics) to refer to non-sibilant fricatives only. The latter can be used synonymously with "sibilant", but some authors include also labiodental, lateral or uvular fricatives in the class.
