Υ υ
Twentieth letter of the
Greek alphabet, ancestor of the Roman letters
U,
V,
W, and
Y. It is usually rendered with Y in borrowings from Greek, as in
psyche and
symphonic and
hyoid. In scholarly transcriptions of
Ancient Greek it is often rendered with U, as in
psukhê.
In Classical Greek it was pronounced [y] like the German Ü, French U, Finnish Y. This could be long or short. Later it changed to an [i] sound, the same as the letters eta and iota, which is how it's pronounced in Modern Greek. Originally it would have been an [u], and possibly still was when the Romans (or Etruscans) started using the Greek alphabet.
It was the first letter added to the Semitic alphabet by the Greeks, which had ended with Τ tau. The Romans used it in the shape V as both a vowel and a consonant (these were only regarded as separate letters in modern times). Later they re-borrowed the Υ-shaped letter as Y to write Greek borrowings. Educated Romans pronounced it with the Greek [y] sound, but after a while it merged with the Roman I. In the Middle Ages it was rounded for the vowel U and doubled up to represent its original consonant value, W. This is how it's the ancestor of all four.
The name means "bare U" and was apparently given to it after it had merged in pronunciation with the diphthong ΟΙ in Byzantine times, but before it had become I (towards the year 1000). You've got a choice about how to pronounce the name. I say it yoop-SI-lon.
An interesting snippet is that initially it always had the rough breathing, that is you only ever get words beginning with hy-, not y-. The reason for this is not clear.
My browser renders the capital upsilon at the top of this node with what looks identical to a Roman Y. A carefully written upsilon has a slight outward curve on its upper parts, which curl round at the top rather than having serifs. The lower-case upsilon curls slightly to the left at the top of both uprights. In a poorly rendered Greek font it may be hard to tell from a lower-case nu, ν, which also curls leftward on both arms, but has a pointed bottom.