Z-index
Z-index: z-index controls the stacking order of positioned and certain other elements within stacking contexts.
What Z-index means
z-index controls the stacking order of positioned and certain other elements within stacking contexts. A larger z-index does not automatically place an element above everything on the page; stacking contexts create local ordering boundaries.
In a real stylesheet, this concept participates in the cascade and in the browser’s layout or painting process. Its effect therefore depends on surrounding rules, element structure and the computed values that ultimately reach the rendering engine.
How it is used in web and design work
For interface work, the practical goal is predictable layout with as few special-case overrides as possible. Clear naming, consistent spacing rules and restrained selector complexity make CSS easier to change as a design grows.
In practical work, Z-index should be interpreted according to its technical or design context. The surrounding document structure, software, output medium, browser behavior or publishing workflow determines how the concept is applied. The purpose of the term is to describe a specific part of that process, not to replace the other concepts around it.
Where the term fits
CSS layout is resolved from several interacting systems rather than from isolated declarations. The browser first determines which declarations win through the cascade, computes values, creates boxes, performs layout and then paints the result. Flexbox, Grid, normal flow and positioned layout have different rules, while transforms and stacking contexts can affect visual placement without changing the surrounding document flow.
The most reliable way to diagnose a CSS term is to inspect the element in browser developer tools. The matched-rules and computed-style views show which declaration won, while box-model and layout inspectors expose dimensions, gaps, tracks and alignment. This avoids guessing from the stylesheet alone, especially when inherited values, shorthand properties, media queries or stacking contexts change the final result.
Why the distinction matters
A common mistake is to solve a local layout problem with increasingly strong overrides without first checking document flow, box dimensions, inheritance, specificity and stacking behavior.
Using precise terminology helps designers, developers, editors and clients describe the same problem without mixing separate layers of a project. That is especially useful when a visual issue may actually come from document structure, when a WordPress issue belongs to a theme or plugin, or when an image problem depends on resolution, color or file type rather than styling.
Related glossary terms
The following entries sit close to Z-index in the same workflow or terminology set. The distinction is useful because each term names a separate concept:
Media Query: A media query conditionally applies CSS according to characteristics of the output environment or viewport.
Positioning: CSS positioning controls how an element participates in normal flow and how offsets are calculated.
Pseudo-class: A CSS pseudo-class selects an element based on state, position or another condition that is not expressed as a normal element attribute.
CSS Grid: CSS Grid is a two-dimensional layout system for arranging content in rows and columns.
Sources and further reading
This entry is a concise, independently written definition based on established documentation. The linked references provide standards, product documentation or broader technical context.