Definition of Terms
A field is the whole Sudoku grid.
A cell is the smallest sub divison of a Sudoku:
Block or 3x3 Sub Grid
A "block" or "3x3 sub grid" is a square of three times three cells. The upper left corner has to be in the first, fourth or seventh row and in the first, fourth or seventh column:
An entity is a row, column or a block.