A solved cell is one whose final digit has been determined - either because the puzzle setter printed it as a given or because the solver placed it through deduction. Tracking solved cells is how you measure progress.
A solved cell is any cell with a confirmed final digit. There are exactly two ways a cell becomes solved: it was printed as a given, or you placed a digit there during solving and that placement is correct.
Note that "solved" in this strict sense means correct - not "I wrote a digit here and hope it sticks." Pencil marks, candidates, and uncertain placements do not count. A naked single placement, however, is a solved cell because it follows from logic.
Each solved cell broadcasts an elimination across its three constraints (row, column, box). Knowing a cell is solved with digit 7 means every other cell in its row, column, and box loses 7 from its candidate list. That single placement often unlocks hidden singles elsewhere, which solve more cells, which broadcast more eliminations - the chain reaction that makes Sudoku rewarding.
Most digital Sudoku apps colour solved cells differently from givens, partly to let you reset only your placements. On paper, solvers usually write givens in pen and placed digits in pencil so corrections do not destroy the original puzzle.
For the underlying mechanics, see givens, cells, and the family of single-placement strategies that produce most solved cells in easy and medium puzzles.
Givens are a subset of solved cells. Every given is solved (its digit is final), but not every solved cell is a given - many solved cells are placed by the solver during play.
It is not actually solved, even if you wrote it confidently. Sooner or later it will conflict with another placement. Mark uncertain digits with pencil marks or notes until the deduction is airtight.
No. Once a cell is solved, its candidates are removed and the placed digit is removed from every other cell in its row, column, and box. Most apps do this automatically.
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