Abstract diction occurs when the poet wants to express something ephemeral, or ungraspable. It can be perceived with the senses or emotions. This could mean it is an idea, such as freedom, or it might be a memory, happy or sad. An example can be found in ‘The Freedom of the Moon’ by Robert Frost .
put it shining anywhere I please.
By walking slowly on some evening later,
I’ve pulled it from a crate of crooked trees,
And brought it over glossy water, greater,
And dropped it in, and seen the image wallow,
The color run, all sorts of wonder follow.
In these lines Frost writes about human freedom by describing the speaker’s ability to pick up the moon, walk with it, and place it where he likes.