Just as with other punctuation marks, there are specific rules for the use of italics. However, years ago, when all we had were typewriters for the everyday person to create a typed paper, there was no way to create italics. We could use a typewriter to create underlining, and underlining became the substitute for italics. This tradition continues to this day. Some publishers are requiring one as opposed to the other, but in many cases they are interchangeable except that the choice should be used consistently within a document. MLA has recently changed the rule that italics only should be used. Do not underline in MLA.
Italics are used for names of long, published works such as magazines, journals, newspapers, and websites:
Science
Academic Search Complete
St. Petersburg Times
Ask.com.
Italics are also used for names of artwork or for names of ships and aircraft
Mona Lisa
U.S.S. Enterprise
Enola Gay
Italics are used for words and letters used as words and letters:
She learned her ABCs.
There are too many fours in that address.
Italicize a word when referring to that word, especially when introducing or defining terms.
The term organic was coined in 1939.
The word Nazi is often overused.
Italics are used for foreign words and phrases. Since so many foreign words have been incorporated into English and many are now considered Standard English words, it is hard to tell what words are considered foreign. A standard dictionary will indicate if you are in doubt.
Use italics for foreign words; use quote marks around the translation.
The Russian word krasnaya can mean either “red” or “beautiful.”
Mano-a-mano is a Spanish construction meaning “hand to hand.”
Very rarely, italics are used for emphasis. Avoid this in research writing.