A metaphor is a figure of speech in which a word or phrase describes something it does not denote. A metaphor establishes a comparison between the figurative language used and the word or phrase being described without explicitly mentioning that to which the metaphor refers.

Metaphor is one of the most common literary devices, particularly in poetry. Writers use metaphors to expand or alter a word's meaning and enhance the imagery. 

Similes and metaphors are both used to create imagery through comparison, but there is a crucial difference between them. A simile makes a comparison explicit, mentioning both the figurative language and the literal thing it describes and connecting them with words such as "like" or " as." A metaphor compares things implicitly without directly stating what is being described. 

Metaphor derives from the Greek word metapherein, meaning " to transfer," formed from meta ("over, across"") and herein ("to bear, to carry").

Metaphors may be found in Emily Dickinson's poem, "The Moon is distant from the Sea – (387):"," makes use of metaphor:

The Moon is distant from the Sea –
And yet, with Amber Hands –
She leads Him – docile as a Boy –
Along appointed Sands –

In the first stanza, Dickinson uses metaphor to describe how the Moon's gravity causes tides. Because the poem does not explicitly mention tides, instead referring to them as "amber hands" leading the Sea, this is an example of a metaphor. This stanza also contains a simile within the metaphor—" docile as a boy"—which can be identified by its use of " as" to connect "docile" and " boy." 

see: analogy, connotation, denotation, simile