From Halopedia, the Halo wiki
Slip Termination, Preventable (or STP for short) is a Shaw-Fujikawa Translight Engine failure classification. STP usually results from poor drive maintenance.
It was due to a slip termination that the freighter Horn of Plenty was attacked by the Covenant missionary vessel Minor Transgression. This was one of the events that started the Human-Covenant War.
Some human freighter captains have their own way of translating the acronym: "Screwing The Pooch."