A bilabial fricative is a phone whose place of articulation is bilabial (with both lips) and whose manner of articulation is fricative.
There are two bilabial fricatives, neither of which appears in standard English. The voiced bilabial fricative (IPA: ) is a sound similar to English v, but with the air going between the lips. It is used for the Spanish letters b and v, as well as in other languages. The voiceless bilabial fricative (IPA: ) is a sound similar to English f, but also made between the lips. It appears in the Japanese sound romanized as f or h, as well as in other languages. The two sounds, as well as the two labiodental fricatives, are used contrastively in Ewe.
George Carlin incorrectly used the phrase bilabial fricative to refer to the sound of blowing a raspberry, which is correctly called a linguolabial trill (except as performed by chimpanzees).
Jack Black also incorrectly used the phrase bilabial fricative in the special features of the Tenacious D in The Pick of Destiny DVD. He was actually referring to a voiced bilabial trill, often used as part of his vocal warm up.