Two or more beers from the same mash. Usually a stronger beer comprised of higher gravity first wort, and weaker beer from subsequent sparge runnings.
Old UK technique. Make a Wee Heavy and then a Scottish Light, or a Barleywine then a Bitter.
Blending various gyles (wort runnoffs) is the most work but gives the most predictable, best tasting results.
Say one stronger beer gets 90% of first wort, 10% of second wort and 50% of third wort, and the second weaker beer gets 10% first wort, 90% second wort, and 50% third wort. That's how I used to do it.