Patter is a prepared and practiced speech that is designed to produce a desired response from its audience. Examples of occupations with a patter might include the auctioneer, salesperson, dance caller, magician, or comedian.
From this, it became a slang word for the secret and equally incomprehensible mutterings of a cant language used by beggars, thieves, gypsies, etc., and then the fluent plausible talk that a cheap-jack employs to pass off his goods. Many illusionists, e.g., card magicians, use patter both to enhance the show and to distract the attention of the spectators.
It is thus also used of any rapid manner of talking, and of a patter-song, in which a very large number of words have to be sung at high speed to fit the music. A western square dance caller may interpolate patter—in the form of metrical lines, often of nonsense—to fill in between commands to the dancers.
In some circumstances, the talk becomes a different sense of "patter": to make a series of rapid strokes or pats, as of raindrops. Here, it is a form of onomatopoeia.
In hypnotherapy, the hypnotist uses a 'patter' or script to deliver positive suggestions for change to the client.