Meridian is a city located in Ada County, Idaho, United States. The population was 117,635 at the 2020 census, making it the 2nd most populous city in the county and Idaho after Boise, the state capital. Meridian is considered the state's fastest-growing city and among the fastest-growing cities in the United States.

Ada County is located in the southwestern part of Idaho, United States. As of the 2020 census, the county had a population of 494,967, which by 2022 was estimated to have risen to 518,907. Ada County is by far the state's most populous county; it is home to 26.8% of the state's population. The county seat and largest city is Boise, which is also the state capital. Ada County is included in the Boise metropolitan area. The Ada County Highway District has jurisdiction over all the local county and city streets, except for private roads and state roads. In the interior Pacific Northwest east of the Cascade Range, Ada County ranks second in population, behind Spokane County, Washington.