Self-invested personal pension
A self-invested personal pension (SIPP) is the name given to the type of UK government-registered personal pension scheme which allows individuals to make their own investment decisions from a wide range of investments by HM Revenue and Customs (HMRC). SIPPs are "tax wrappers", allowing tax rebates on contributions in exchange for limits on accessibility.
Source: Wikipedia — Self-invested personal pension (CC BY-SA 4.0)