Definition of self-invested personal pension

As the name suggests, a sipp (self-invested personal pension) is a type of do-it-yourself personal pension where you pick the investments. It's not actually a product, but a tax wrapper into which you put investments. Like other pensions, there is no Capital Gains Tax to pay on profits and you can take 25 per cent of the pension fund as a tax-free lump sum on retirement. As you'll be controlling your own retirement nest egg, you need to be a reasonably experienced investor to take out a sipp. [1]

Sipps vs personal pensions
Weird and wacky Sipp investments