Personally, I call anything that needs cell values to be from 1-9 [or whatever the puzzle says] a "region" for the sake of uniformity and simplicity. I would call any region that isn't commonly named (row, column, box, disjoint-group, diagonal) simply a region.
Of course, the region idea can accomodate any disjoint group you'd care to think of. It can be useful in programming (if you want your program to have a LOT of extra variations, for example) to simply define all rows, columns, boxes, irregular regions, diagonals, etc. as "region"s in a big array and let the program do the rest. (I had to do that when I put 6-dimensional and NRC constraints in mine.)
