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 …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.