
1) Greek letter
Found on
https://www.crosswordclues.com/clue/digamma

Digamma (or wau, uppercase Ϝ, lowercase ϝ; as a numeral: stigma, ϛ) is an archaic letter of the Greek alphabet, which originally stood for the sound w and later remained in use only as a symbol for the number `6`. Whereas it was originally called wau, its most common appellation in classical Greek is digamma, while, as a numeral, it was calle.....
Found on
http://en.wikipedia.org/wiki/Digamma

• (n.) A letter (/, /) of the Greek alphabet, which early fell into disuse.
Found on
http://thinkexist.com/dictionary/meaning/digamma/

(from the article `Bentley, Richard`) ...which were published after his death. Bentley made a particularly important scholarly contribution through his discovery that a sound (represented ... ...also substantially later than the foundation of the first Ionian settlements in Asia Minor of about 1000 . The running together of adjacent sho...
Found on
http://www.britannica.com/eb/a-z/d/48
Di·gam'ma noun [ Greek ...; ... = ... twice +
ga`mma the letter Γ. So called because it resembled two
gammas placed one above the other.]
(Gr. Gram.) A letter (..., ...) of the Greek alphabet, which early fell into disuse. » This form identifies it with the Latin F,...
Found on
http://www.encyclo.co.uk/webster/D/64

Digamma was an ancient Greek letter, which was already obsolete in classical times and was longest in use among the Aeolians. It was so called because its form resembled a double gamma. It was pronounced like an English w.
Found on
http://www.probertencyclopaedia.com/browse/AD.HTM

digamma A letter occurring in certain early forms of Greek and transliterated in English as 'w'. From Latin, which came from Greek di-, 'two' + gamma, 'gamma' (because its shape resembles two gammas).
Found on
http://www.wordinfo.info/words/index/info/view_unit/882/

a letter of the early Greek alphabet that generally fell into disuse in Attic Greek before the classical period and that represented a sound similar to English w.
Found on
https://www.infoplease.com/dictionary/digamma
No exact match found.