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. [1913 Webster]
hello, int., n., & v. (also hallo, hullo)
1 a an expression of informal greeting, or of surprise. b used to begin a telephone conversation.
2 a cry used to call attention.
--n. (pl. -os) a cry of 'hello'.
--v.intr. (-oes, -oed) cry 'hello'.