From Sudopedia, the free Sudoku reference guide

From Sudopedia

A peer or buddy is a cell that shares one or more houses with another cell.

In standard Sudoku, each cell has 20 peers—3 houses (box / column / row) each containing 8 peers, less 4 that overlap: ((3 x 8) - 4) = 20. This image shows the peers of r5c5:

In Killer Sudoku, any two cells are also peers of each other if they share the same cage, even if they do not share a house. This is the so-called Killer Convention.

Peers play an important role in solving techniques, because peers cannot contain the same digit.

Note

Another way of saying that cells X and Y are peers is to say "X sees (or intersects) Y".