A letter in the Belarusian language, when written in the Latin alphabet (as was normal from the 16th to late 19th centuries and is slowly reemerging since the collapse of the Soviet Union). The letter is pronounced with a consistent sound, represented by [w] in SAMPA. When Belarusian is written in the Cyrillic alphabet, the same letter is written as "ў".
A semivowel and letter in the Esperanto alphabet, which was devised in the late 19th century. (In terms of a word's syllable structure, the nature of a semivowel is more like a consonant than a vowel, in that it does not form the heart of a syllable; a semivowel combines with a true vowel to form a diphthong.) It too is pronounced consistently as [w] in SAMPA.
It is almost certain that the Esperanto letter was taken from the Belarusian, because:
Belarusian is the only natural language whose orthography contains this letter.
The letter has the same pronunciation in the two languages.