Symbol/symbolism:
Generally, anything that represents, stands for, something else. Usually a symbolism is something concrete -- such as an object, character, action or scene--

Example:
"Much I marvelled this ungainly fowl to hear discourse so plainly,
Though its answer little meaning - little relevancy bore;
For we cannot help agreeing that no living human being
Ever yet was blessed with seeing bird above his chamber door -
Bird or beast above the sculptured bust above his chamber door,
With such name as `Nevermore.'

But the raven, sitting lonely on the placid bust, spoke only,
That one word, as if his soul in that one word he did outpour.
Nothing further then he uttered - not a feather then he fluttered -
Till I scarcely more than muttered `Other friends have flown before -
On the morrow he will leave me, as my hopes have flown before.'
Then the bird said, `Nevermore.'"

-The Raven: by Edgar Allen Poe

Explanation:

The first, and most obvious symbol is the Raven. He uses it as a symbol for death, which is what the entire poem is devoted to. Poe does this to create a vivid symbol that is easier for the reader to detect.  Another symbol is the bust of Pallas, the reason for this is because it Pallas is the God of wisdom, which makes the narrator think that the raven is speaking from wisdom. These symbols allow the reader to makes inferences without coming right out and saying it. This is one purpose, but also it adds a hidden layer of depth to the poem, showing its sophistication.