A Unique Rectangle is a Deadly Pattern formed by 4 cells that share 2 rows, 2 columns and 2 boxes. In Sudoku Variations with additional constraints, each of these constraints must either contain 2 cells or none at all. All 4 cells only have candidates for the same 2 digits.
The name is not well chosen. The pattern causes the puzzle to have at least 2 solutions, so a better name would be non-unique rectangle.
Here is an example:
Rows 1 and 2, columns 1 and 4 and boxes 1 and 2 each contain 2 cells from this pattern. All cells have candidates 1 and 2. There are 2 ways to complete this pattern.
A more concrete example:
This puzzle has two solutions.
A mistake to be avoided
The following pattern is sometimes mistaken for a unique rectangle. However, this pattern occupies 4 boxes.
This is not a unique rectangle!