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A variable part of a URL that lets one route handle many different items — like a mail slot labeled 'Apartment ___' that accepts any number.
A named placeholder in a route path (e.g., /users/:id) whose value is extracted from the URL and available on req.params. Route parameters make URLs dynamic and RESTful without creating separate routes for each resource.
Express compiles route patterns to regular expressions via path-to-regexp. Named parameters (:param) capture a single path segment; modifiers (?, *, +) control optionality and repetition. The router matches routes in registration order, and req.params is populated per the first match. Param middleware (app.param()) can pre-process parameter values for validation or database lookups before the route handler executes.