A statistically improbable phrase (SIP) is a string of words having a low probability of being encountered in normal English discourse, and more importantly, in the use of computer searches for text or sales items.

Amazon added a search function called SIPs in mid-2005. New books have a list of associated SIPs so that subsequent searches can be quicker.

Dan Brown's recent bestseller, The Da Vinci Code, for example, has the following SIPs:

- cilice belt
- lame saint
- seeded womb
- lettered dials
- corporal mortification
- rosewood box
- sacred feminine
- royal bloodline
- stone cylinder
- sweater pocket

I first encountered SIPs when I did a search for A. Zee's Quantum Field Theory in a Nutshell. (A page turner, trust me. *koff*) Its SIPs are a little less... vanilla, one might say:

- meson meson scattering amplitude
- double line formalism
- hermitean traceless matrices
- electromagnetic gauge potential
- effective field theory approach
- anyon statistics
- nonabelian gauge theory
- experimentalist friend
- graviton propagator
- renormalization group flow
- gapless mode
- massless gauge field
- nonrenormalizable theories
- spin statistics connection
- gauge boson propagator
- graviton field
- dimensional field theory
- field gyp
- hermitean matrices
- scalar field theory
- fractional statistics
- propagate from the source
- harmonic gauge
- massive spin
- gamma matrices