An emoji is the name for an image/drawing which represents a single specific unicode character, not a series of characters. The name comes from Japanese e (絵, "picture") + moji (文字, "character") [1]

Multi-character ones would be called smileys, emotes, or emoticons. The confusion comes from the fact that many messengers will automatically replace multi-character emotes with an emoji.

[1] https://en.wikipedia.org/wiki/Emoji