The symbol # means number. As for the apostrophe ('), it's used to form plurals, especially the plural of letters, digits, and symbols.Originally Posted by jack
MSN & ICQ #'s => MSN & ICQ numbers
MSN & ICQ #s => MSN &ICQ numbers
An apostrophe is also used to form some plurals, especially the plural of letters and digits. Raoul got four A's last term and his sister got four 6's in the ice-skating competition. This is particularly useful when the letter being pluralized is in the lower case: "minding one's p's and q's" or "Don't forget to dot your i's." (In a context in which the plural is clear, apostrophes after upper-case letters are not necessary: "He got four As, two Bs, and three Cs.") It is no longer considered necessary or even correct to create the plural of years or decades or abbreviations with an apostrophe:
He wrote several novels during the 1930s.
There are fifteen PhDs on our faculty.
My sister and I have identical IQs.
(If you wrote Ph.D. with periods, you would add an apostrophe before the pluralizing "s": Ph.D.'s) If the abbreviation or acronym ends in "S," it's a good idea to separate this final "S" from the pluralizing "s" with an apostrophe: SOS's