alkali metal

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of a group of soft, white, low-density, low-melting, highly reactive metallic elements, including lithium, sodium, potassium, rubidium, cesium, and francium.

Wiktionary

1. n. inorganic chemistry Any of the soft, light, reactive metals of Group 1 of the periodic table; lithium, sodium, potassium, rubidium, cesium, and francium.

GNU Webster's 1913

1. Any one of the univalent metals of group I of the periodic table of the elements, including lithium, sodium, potassium, rubidium, cesium, and francium. The hydroxides of these metals are soluble in water and form strongly basic solutions.

WordNet 3.0

1. n. any of the monovalent metals of group I of the periodic table (lithium or sodium or potassium or rubidium or cesium or francium)