The letter Y is the twenty-fifth and penultimate letter in the basic modern Latin alphabet, a vowel. Its name in English (pronounced /ˈwaɪ/) is spelt wye or occasionally wy, plural wyes.
The ancestor of Y was the Semitic letter waw, from which also come F, U, and W. See F for details.
When pronunciation of upsilon in the prestigious Attic dialect changed to /y/, the Romans borrowed it again as Y to write Greek loan-words. Y was named Y Graeca "Greek Y". This was no doubt pronounced as I Graeca "Greek I", since Latin speakers had trouble pronouncing /y/, which was not a native sound. In Romance languages, the pronunciation became the regular name: Spanish i griega, French i grec, etc.
By the time of Middle English, /y/ had lost its roundedness and became identical to I (/iː/ and /ɪ/). Therefore, many words that originally had I were spelled with Y, and vice-versa. A similar substitution occurred in Latin words: original silva "wood" is spelled with Y in Pennsylvania.
Likewise, Modern English vocalic Y is pronounced identically to I. But Modern English uses it in only certain places, unlike Middle and early Modern English. It has three uses: for upsilon in Greek loan-words (system: Greek σύστημα), at the end of a word (rye, city; compare cities, where S is final), and in monosyllabic stems before vowel endings (dy-ing).
The consonantal use of Y for /j/ (year, German Jahr) is probably unrelated to vocalic use. Perhaps it was a typesetters' substitution for the Middle English letter yogh (Ȝȝ) where it represented /j/. Yogh representing the letter's other sound, /ɣ/, came to be written gh in Modern English.
In Spanish, Y is called i/y griega, in Catalan i grega, in French and Romanian i grec, in Polish igrek - all meaning "Greek i" (except for Polish, where it is simply a phonetic transcription of the French name); in most other European languages the Greek name is still used; in German, for example, it is called Ypsilon and in Portuguese and Italian it's called ípsilon or ípsilo (although in Portuguese there is also the name "Greek i"). The letter Y was originally established as a vowel. In the standard English language, the letter Y is traditionally regarded as a consonant, but a survey of almost any English text will show that Y more commonly functions as a vowel. In many cases, it is known as a semivowel.
In English morphology, -y is a diminutive suffix.
In Dutch, Y appears only in loanwords and names and usually represents /i/. It is often left out of the Dutch alphabet and replaced with the "ligature IJ". In Afrikaans, a development of Dutch, Y denotes the diphthong [ɛi], probably as a result of mixing lower case i and y or may derive from the IJ ligature.
Appearing alone as a word, the letter Y is a grammatical conjunction with the meaning "and" in Spanish and is pronounced /i/. In Spanish family names, y can separate the father's surname from the mother's surname as in "Santiago Ramón y Cajal"; another example is "Maturin y Domanova", from the Jack Aubrey novel sequence. Catalan names use i for this. Otherwise, Y represents /ʝ/ in Spanish. When coming before the sound /i/, Y is replaced with E: "español e inglés". This is to avoid pronouncing /i/ twice.