I hear a lot of people saying "Send me an invite". I always thought that it was an 'invitation'. Is "sending one an invite" accepted usage? Or is it incorrect? If I need to get my wedding invitation printed, should it read "Wedding Invitation" or a "Wedding Invite"? Is this a US/UK usage difference?
Invitation is the more accepted noun to use.
Using invite as a noun is informal. I've found it prevalent in the U.S. and internet culture, though it's growing here in the U.K. too.
I suspect that the growth in usage comes from internet applications, particularly the developer-driven Facebook. Developers don't like to type or read any more text than they need to, and invitation is longer and harder to spell. We're so lazy. I apologise.
Merriam-Webster's Collegiate Dictionary, editions 9, 10, and 11 (1983, 1993, and 2003) have this entry for invite as a noun:
invite n (1659): INVITATION 1
where "invitation 1" can mean "a : the act of inviting b : an often formal request to be present or participate." The 1659 date almost certainly refers to the Hamon L'Estrange instance that Barrie England cites. From Hamon L'Estrange, Alliance of Divine Offices (1659):
Being thus necessitated to abandon Strasburgh, he [Martin Bucer] intimated his condition to a friend of his in England, that Friend acquaints the Bishop Cranmer therewith, who presently by an express of his own, dated October 2. 48. and after by his secretary Peter Alexander, March 14. 49. gives him an earnest invite to England, with promises of ample promotion.
Editions 7 (1963) and 8 (1973) of the Collegiate Dictionary, however, include a significant additional label in their definition of the term:
invite n, chiefly dial: INVITATION 1
Earlier editions (1 through 6) of the Collegiate Dictionary have no entry for invite as a noun. Indeed, many English and American dictionaries fail to acknowledge the existence of invite as a noun in the period between 1756 and 1961: Johnson's Dictionary of the English Language (1756); Webster's Compendious Dictionary of the English Language (1806); Webster's American Dictionary of the English Language (1828); Worcester's Comprehensive Dictionary (1845); Webster's American Dictionary of the English Language (1847); Webster's American Dictionary of the English Language (1864); Worcester's Comprehensive Dictionary (1874); Webster's International Dictionary (1890); Webster's New International Dictionary (1909); and Webster's Collegiate Dictionary, First through Sixth Editions (1898, 1910, 1916, 1931, 1936, and 1949).
More-recent dictionaries do list invite as a noun, but often with usage labels that don't indicate unqualified support for using it in every situation where "invitation 1" might be appropriate. The Oxford Universal Dictionary (1955), for example, characterizes the noun form of invite as "colloq. or vulgar." Simon & Schuster's Webster's New Twentieth Century Dictionary (1983) labels it "Slang." The Random House College Dictionary (1984) calls it "Chiefly Dial." And The American Heritage Dictionary of the English Language, First, Edition (1969), Third Edition (1992), and Fourth Edition (2000); Encarta World English Dictionary (1999); The New Oxford American Dictionary (2001); and The Concise Oxford English Dictionary (2002) all list it as "informal."
This record suggests three things: First, if invite as a noun was popular prior to Samuel Johnson's day, it went into so severe an eclipse over the 200-year period that followed his dictionary that (Fanny Burney's endorsement in 1778 notwithstanding) of the many standard dictionary published between 1756 and 1961 that I consulted, only The Oxford Universal Dictionary included it all. Second, when it did begin to pop up in everyday, single-volume dictionaries starting in the 1960s, it did so with the explicit observation that it was a colloquial, vulgar, chiefly dialectal, slang, or informal term. Third, even in the crop of dictionaries that emerged in the period between 1999 and 2003, only Merriam-Webster's dictionary lists it without the label "informal."
As invite as a noun grows more and more common in everyday use, Merriam-Webster's decision to treat it as simply a standard English word (that is, as a word whose appearance in the dictionary doesn't call for a clarifying usage label) moves closer and closer to vindication. But at this point, invite hasn't yet become completely interchangeable with "invitation 1."