Undo

Undo is an operation in computer software that reverses the effects of one or more prior actions, restoring the state of a document or application to a previous point. It is typically provided as part of an undo/redo mechanism that records changes in a history stack or via periodic snapshots. When the user triggers undo, the system applies the inverse of the most recent action or loads the previous snapshot, moving that state onto a redo stack so the change can be reapplied if desired.

Most systems implement undo with a stack of actions. Each reversible action stores enough information to reverse itself. Some systems store full state snapshots for larger changes but consume more memory. Actions can be grouped into a single undo unit, so a sequence of edits can be undone together.

Common interfaces include keyboard shortcuts such as Ctrl+Z or Cmd+Z to undo, and Ctrl+Y or Cmd+Shift+Z to redo. Menus labeled Undo and Redo provide alternate access.

Limitations include that not all actions are undoable; some irreversible operations may permanently discard data. Undo history may be lost on closing an application unless a persistent history is implemented, and memory or performance constraints can limit how much history is kept.

In collaborative editing, undo can be complex because concurrent edits must be reconciled. Some systems use techniques such as operational transformation or conflict-free replicated data types to maintain consistent histories.

Beyond editors, the concept also appears in databases and version control, where transactions or commits provide a way to revert to earlier states. See also redo, version history.