An Alem is a type of finial. In Arabic (أليم) and Turkish, the word alem means a flag, banner or standard (an emblem atop a staff or pole); by extension the word came to be applied to the decoration at the top of a flagpole.
After the rise of Islam (and especially after the fall of Constantinople to the Ottomans in 1453) the word alem also came to be applied to the architectural device which caps a minaret or the dome of a mosque. It may incorporate an Islamic emblem of the crescent (with star), a tulip or other shape.