- Used to express strong emotion, such as surprise, fear, anger, or pain. (interjection)
- Used in direct address: Oh, sir! You forgot your keys. (interjection)
- Used to indicate understanding or acknowledgment of a statement. (interjection)
- Zero. (noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.