hello

American Heritage® Dictionary of the English Language, Fourth Edition

1. interj. Used to greet someone, answer the telephone, or express surprise.
2. n. A calling or greeting of "hello.”
3. v. To call "hello.”

Century Dictionary and Cyclopedia

1. An exclamation designed to attract the attention of a person at a distance; also, a mere greeting between persons meeting. As a greeting its use is confined to easy colloquial or vulgar speech. As a preliminary telephone call it is now (1889) in very common use.

Wiktionary

1. interj. A greeting (salutation) said when meeting someone or acknowledging someone’s arrival or presence.
2. interj. A greeting used when answering the telephone.
3. interj. A call for response if it is not clear if anyone is present or listening, or if a telephone conversation may have been disconnected.
4. interj. Used sarcastically to imply that the person addressed or referred to has done something the speaker or writer considers to be foolish.
5. interj. An expression of puzzlement or discovery.
6. n. "Hello!" or an equivalent greeting.

GNU Webster's 1913

1. interj. An exclamation used as a greeting, to call attention, as an exclamation of surprise, or to encourage one. This variant of halloo and holloo has become the dominant form. In the United States, it is the most common greeting used in answering a telephone.

WordNet 3.0

1. n. an expression of greeting