A function which handles a Request.
For example a static file handler may read the requested URI from the filesystem and return it as the body of the Response.
A Handler which wraps one or more other handlers to perform pre or post processing is known as a "middleware".
A Handler may receive a request directly from an HTTP server or it may have been touched by other middleware. Similarly the response may be directly returned by an HTTP server or have further processing done by other middleware.
typedef Handler = FutureOr<Response> Function(Request request);