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: