The combobox role - ARIA Reference Guide

Composites
Represent containers for navigating interactive descendants.
Landmarks
Represent top-level sections of a page, relevant to a specific purpose, that a user may want to scan for quickly and could appear in a site map or table of contents.
Live Regions
Represent dynamic content or announcements, typically updated as a result of an external event when user focus may be elsewhere.
Structures
Represent non-interactive parts of a page that organize content into coherent units.
Widgets
Represent focusable, interactive elements.
Windows
Represent inner windows that are children of the main web document.

WAI-ARIA spec HTML-ARIA spec

Widget(opens in new window)Interactive Content(opens in new window)Phrasing Content(opens in new window)

Description

An input that controls another element, such as a listbox or grid, that can dynamically pop up to help the user set the value of the input.

Allowed Content

Flow content but with no main element descendants.

HTML Elements with Implicit ARIA Role

select

Allowed HTML Elements

button
input[type=text]
select (role attribute unnecessary)
div, span, p, other elements that can receive any role