Three Mile Island

1. An island of Londonderry Township, Dauphin County, Pennsylvania, United States, on Lake Frederick, Susquehanna River; the site of a nuclear power plant accident in 1979.
2. A nuclear power plant located on the island; the site of a nuclear accident in 1979.
3. A nuclear reactor core partial meltdown in 1979, at the nuclear power plant's reactor unit 2
4. An island of the town of Meredith, Belknap County, Lakes Region, New Hampshire, United States, on Lake Winnipesaukee.

plural Three Mile Islands

1. An informal unit of measurement for a nuclear disaster, equivalent to that of the incident in 1979 in Pennsylvania