As

As is a word used to compare the similarities of two ideas. Like "like", it is a simile. But as is more often used in idioms specific to English:

• "Green as grass"
• "Right as rain"

Usually, one idea is more abstract and the other is more concrete - and "as" is intended to be more general than just one situation.

• "Love as war"
• "Time as a path"

In other words, you can say more generally what an idiom or metaphor says: for example, "battle of the sexes" is both a metaphor and an idiom that suggests "love as war".

Because of this power, you can state an idiom or metaphor in a way that makes it clear what it means, and what it might assume.