E-how UK has this definition:
The tri-tip is a triangular cut of beef from the tip of the cow's sirloin. It is a thick, rich, flavourful steak, popular for barbecue grilling and oven roasting. However, tri-tips can be difficult to find -- the cut goes by many names, and your local butcher may not know what a tri-tip is. Some butchers grind this part of the cow into mince or slice it into steaks rather than sell it whole, and some do not regularly stock it at all. However, most will supply it on request.
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