Your complete reference for every Akari and Light Up puzzle term you need to know.
Welcome to the GridPaw akari glossary — a comprehensive reference for every term you will encounter while solving Akari (Light Up) puzzles. Whether you are a beginner learning your first akari puzzle or an experienced solver brushing up on advanced techniques, this puzzle glossary covers all the essential akari terms in one place. Bookmark this page and return whenever you come across unfamiliar terminology.
A number cell is a black cell in an Akari puzzle that displays a digit from 0 to 4. This number tells you exactly how many light bulbs (or cats, on GridPaw) are placed in the four orthogonally adjacent cells — up, down, left, and right. Number cells are the primary source of logical constraints in every Akari puzzle. A "0" means none of its neighbors can hold a bulb, while a "4" means all four neighbors must contain one. Mastering number cell logic is the first step to solving any Akari puzzle.
A bulb (also called a cat on GridPaw) is the object you place on white cells to illuminate the grid. Each bulb sends light in four directions — up, down, left, and right — until the light hits a black wall or the grid edge. The goal of every Light Up puzzle is to place bulbs so that every white cell is illuminated. On GridPaw, bulbs are replaced with adorable glowing cats, but the mechanics are identical. Learn more about placing bulbs in our how to play guide.
To light up or illuminate a cell means that at least one bulb's line of sight reaches it. When you place a bulb, it illuminates every white cell in its row and column until blocked by a wall. The win condition of every light up puzzle requires that all white cells are illuminated simultaneously. Cells that are not reached by any bulb remain "dark" and indicate that the puzzle is not yet solved.
An X mark is a player-placed annotation on a white cell to indicate that no bulb should go there. X marks are not part of the official Akari rules — they are a solving aid. Most online Akari implementations, including GridPaw, let you place X marks by right-clicking or long-pressing a cell. Using X marks helps you track elimination logic and avoid accidentally placing bulbs in impossible positions. See our how to play guide for more on using X marks effectively.
A black wall is any dark cell in the grid that blocks light propagation. Black walls come in two types: plain walls (no number) and numbered walls. Plain walls simply block light — they carry no adjacency constraints. Numbered walls, on the other hand, enforce a specific bulb count on their neighbors. Understanding the difference between these wall types is fundamental to solving Akari puzzles. Read the complete Akari rules for details.
Elimination is the process of ruling out impossible bulb placements based on logical deduction. When you determine that a cell cannot contain a bulb — because it would violate a number constraint, create a bulb conflict, or leave another cell permanently dark — you eliminate it. Elimination is the bread and butter of Akari solving: every step forward comes from either placing a bulb or eliminating a position. Advanced solvers use systematic elimination chains to crack hard puzzles. We will cover elimination strategies in detail in our upcoming tips and strategies guide.
Constraint propagation is the foundational solving technique in every Akari puzzle. It works by examining numbered cells and deducing forced placements or eliminations. For example, a "4" cell forces all four neighbors to be bulbs, while a "0" cell eliminates all four neighbors. After each placement, you propagate the consequences — newly placed bulbs illuminate rows and columns, which may trigger further deductions. Constraint propagation is what the GridPaw Akari solver uses as its first pass on every puzzle.
backtracking is an advanced solving technique — and the method computer solvers use when simple deduction is not enough. You pick an undecided cell, assume it holds a bulb, and propagate all consequences. If you reach a contradiction, your assumption was wrong and that cell cannot hold a bulb. If no contradiction appears, you continue until the puzzle is solved. Human solvers rarely need formal backtracking on Easy or Medium puzzles, but Hard puzzles on larger grids may require it. The GridPaw Akari solver uses backtracking as its final resolution step.
Akari (あかり) is the original Japanese name for this logic puzzle, meaning "light." It was created and published by Nikoli in 2001. The name reflects the core mechanic: placing light sources to illuminate every dark cell. Akari is one of Nikoli's most popular puzzle types alongside Sudoku, Kakuro, and Slitherlink. Learn more about the puzzle's history and design on our Akari puzzle page.
A light up puzzle is the English name for Akari. The two terms are completely interchangeable — same rules, same logic, same solving techniques. "Light Up" became the standard English localization when Nikoli's puzzles spread internationally through newspapers and online puzzle sites. Some players prefer "Light Up" because it is self-descriptive, while purists and Japanese puzzle fans stick with "Akari." Either way, you are playing the same game. Explore our full light up puzzle guide for rules, strategies, and history.
Nikoli (ニコリ) is the legendary Japanese puzzle publisher that created Akari, Sudoku, Kakuro, Slitherlink, and dozens of other logic puzzles. Founded in 1980, Nikoli's editorial philosophy is strict: every puzzle must have a unique solution solvable through pure logic alone — no guessing, no trivia, no luck. This philosophy makes Nikoli puzzles the gold standard for logic puzzle quality worldwide. Visit Nikoli's official website to learn more about their puzzle catalog.
A daily challenge is a new Akari puzzle released every day for players to solve. Daily challenges are a popular feature across puzzle sites because they create a solving habit and give players a shared experience to discuss. On GridPaw, the daily Akari challenge offers a fresh puzzle each day with a balanced difficulty level suitable for most solvers. Come back every day to keep your deduction skills sharp and build your solving streak.
Understanding akari terms is essential for improving your solving speed and accuracy. When you know what constraint propagation means, you can apply it deliberately instead of stumbling into deductions by accident. When you understand backtracking, you know exactly when to stop relying on simple logic and start testing hypotheses. This puzzle glossary is designed to be your quick-reference companion — keep it open in a tab while you play and look up any term you encounter.
The akari glossary on this page covers every term from beginner concepts like number cells and bulbs to advanced techniques like constraint propagation and backtracking. As the GridPaw site grows, we will continue expanding this akari glossary with new terms, tips, and cross-references to help you become a better solver.
There is no difference. "Akari" is the original Japanese name by Nikoli, and "Light Up" is the English localization. Both terms refer to the same logic puzzle with identical rules. This akari glossary uses both names interchangeably.
No. The essential terms for beginners are number cell, bulb (or cat), black wall, and illumination. The solving technique terms like constraint propagation and backtracking become useful once you move beyond Easy puzzles. Start playing and refer back to this puzzle glossary as needed.
Play the daily Akari challenge on GridPaw for a fresh puzzle every day, or try our how to play guide for a step-by-step introduction. You can also explore the Akari solver to see constraint propagation and backtracking in action.
