Real Dictionary

hey
convention: In informal situations, you say or shout 'hey' to attract someone's attention, or to show surprise, interest, or annoyance.

hey in American English
used to attract attention, express surprise, delight, puzzlement, etc., or preface a remark
interjection: (used as an exclamation to call attention or to express pleasure, surprise, bewilderment, etc.)

hey in British English
exclamation: an expression indicating surprise, dismay, discovery, etc, or calling for another's attention