Short, specific definitions for every term used across this site, including the notation used in technique pages and hints.
Board structure
Row — one of the nine horizontal lines of 9 cells, numbered 1-9 top to bottom.
Column — one of the nine vertical lines of 9 cells, numbered 1-9 left to right.
Box — one of the nine 3x3 blocks the grid is divided into.
Unit — any row, column, or box. Used when a rule applies to all three the same way (“eliminate from the rest of the unit”).
Cell — a single square in the grid, belonging to exactly one row, one column, and one box.
Peer — any other cell sharing a row, column, or box with a given cell. Every cell has 20 peers.
Notation
r4c7 — shorthand for “row 4, column 7,” read left to right: row number first, then column number. This site uses it throughout the technique pages and in-game hints instead of longer phrasing like “the cell in the fourth row and seventh column.”
Values and state
Given — a digit printed on the board at the start of a puzzle, not entered by the player. Givens can’t be changed.
Candidate — a digit that hasn’t yet been ruled out for an empty cell by its row, column, or box. A cell’s candidates shrink as the puzzle fills in.
Pencil mark / note — a candidate written down by the solver (by hand, or in notes mode on this site) as a memory aid, rather than calculated fresh each time. See how to use pencil marks.
Naked — describes a technique based on a cell’s own candidates directly (a “naked single” is a cell with exactly one candidate left).
Hidden — describes a technique based on where a digit can go across a unit, even if the cell itself still shows other candidates. See hidden single.
Difficulty and solving
Technique — a named, specific piece of logic (naked single, X-Wing, and so on) that lets you place a digit or eliminate a candidate with certainty. See the full list on the techniques hub.
Logical solver — a program (ours, specifically) that solves a grid by applying named techniques in order, the way a careful human solver would, rather than by brute-force guessing. Used for both difficulty grading and in-game hints.
Backtracking solver — a different kind of solver that tries digits and reverses course when it hits a dead end, used to check whether a grid has exactly one solution, not to teach technique.
Difficulty band — one of Easy, Medium, Hard, Expert, or Evil on this site, determined by the hardest technique a puzzle’s logical solve requires. See how difficulty is rated.
Unique solution — the property that a properly constructed puzzle can be finished exactly one way. See uniqueness basics.
Puzzle formats
Daily puzzle — a puzzle generated from a seed derived from today’s date, so every player sees the same puzzle on a given day. See the daily puzzle.
Permalink — a shareable link to one specific puzzle, encoded directly into the URL rather than stored in a database.
Killer sudoku — a variant adding labeled “cages” with digit-sum targets on top of the standard rules. See rules and variations.