Symbol

Definition: 
A person, a place, a thing, or an event that stands for something beyond itself.

Example: 

“A Sick Rose” by William Blake, 1794

Rose, thou art sick! 

The invisible worm 

That flies in the night, 

In the howling storm,

Has found out thy bed 

Of crimson joy: 

And his dark secret love 

Does thy life destroy. 

[ This's poem that use rose as the symbol]  

Significant: 

Poets use symbol to indirectly write about something that they want to talk about. It's kind of the same extended metaphor.