Hello

Concise Oxford English Dictionary © 2008 Oxford University Press:
hello /həˈləʊ/ (also hallo or hullo)

▶exclamation

• 1 used as a greeting or to begin a telephone conversation.

• 2 Brit. used to express surprise or as a cry to attract someone's attention.

  ■ used informally to express sarcasm or anger.

– origin C19: var. of earlier hollo; rel. to holla.