Trinity Stitch is one common name for a stitch that is also sometimes called a Cluster Stitch or Bramble Stitch. You might also see it called Raspberry Stitch or Blackberry Stitch.
Whatever you call it, It is formed by working multiple stitches into one stitch, then decreasing the same number of stitches.
The look is kind of puffy and full of texture. It's almost like a bobble, though a bobble is typically made in a different way and usually stands out from the knit fabric a little more prominently than Trinity Stitch does.
The stitch pattern is pretty dense and uses a lot of yarn.
How to Knit Trinity Stitch
Trinity Stitch is worked on a multiple of 4 stitches. The pattern rows are the wrong side of the project, and they stick out onto the right side, which is worked in purl.
Row 1 (right side): Purl.
Row 2: *(K1, p1, k1) all in the same stitch, purl 3 together. Repeat from * across.
Row 3: Purl.
Row 4: *P3tog (k1, p1, k1) all in the same stitch. Repeat from * across.
Repeat these four rows for pattern.
Projects Using Trinity Stitch
I have used Trinity Stitch on a prayer shawl for a baby (it's also a great doll baby blanket, which is how my daughter uses it) and a simple Trinity Stitch Headband. It's a great stitch pattern for fun summery accessories and works great in stripes or color blocking.
If you'd like to explore this stitch more, there are a lot of great patterns available.
Breadnbadger has a great slouchy beret that uses the stitch, worked in the round inside out so there is more knitting than purling.
Another fun hat -- inspired by one Hermione wore in "Harry Potter and the Deathly Hallows, Part One" -- is available from Lindy's Knits and Laces.
Inspired by that first hat, heatherwil worked up a matching pair of mitts, also worked inside out in the round. The Casting On Couch also has video instructions for Trinity Stitch mitts and a scarf (scroll down the page to find them).
Red Heart has a great pattern for mitts, and a matching cowl worked with Trinity Stitch and cables, which look lovely together.
Rag Lana has a pattern for a scarf that uses a variation of the stitch involving a yarn over in the middle of the triple stitch instead of the purl (so you work k1, yo, k1 in one stitch, which is a little easier and doesn't make a difference in the look of the finished stitch). Check that out if you find the traditional form of this stitch too tricky.
Joanna Nassiokas has a lovely, simple, Trinity Stitch neckwarmer (small cowl) that would be great for quick gift knitting.
If you're looking for something a little different, try the Trinity Stitch Shawl from Esther Smith Bozak. This is a triangular shawl worked from the bottom up, starting with just one stitch. This one is worked more like a lace project, with sport weight yarn on larger than expected needles, which really makes the brambles stand out in a different way from the other projects listed here.
Remember you can also search for the name variations when looking for patterns. For example, looking for Raspberry Stitch you will find a cute pair of fingerless mitts with a little contrast on the thumb (from ShortRounds) and a simple cowl form Undeniable Glitter.