In traditional philosophy a 'concrete term' is defined as a word which denotes a particular person or thing, and an 'abstract term' is defined as a noun which denotes qualities that exist only as attributes of particular persons or things.
A sentence, accordingly, is said to be concrete if it makes an assertion about a particular subject, and abstract if it makes an assertion about an abstract subject. With reference to literature, however, these terms are often used in an extended way: a passage is called abstract if it represents its subject matter in general or nonsensuous words or with only a thin realization of its experienced qualities; it is called concrete if it represents its subject matter with striking particularity and sensuous detail. In his Ode to Psyche, John Keats uses a concrete description of a local which involveds qualities that are perceived by four senses; hearing, touch, sight and smell.
'Mid hush'd, cool-rooted flowers, fragrant-eyed, Blue, silver-white, and budded Tyrian
My heart aches, and a drowsy numbness pains My sense, as though of hemlock I had drunk Or emptied some dull opiate to the drains...
It is frequently asserted that 'poetry is concrete', or, as John Crowe Ranson put it in The World's Body(1938), that its proper subject is 'the rich, contingent materiality of things.' Most poetry is certainly more concrete than other modes of language, especially in its use of imagery. It should be kept in mind, however, that poets do not hesitate to use abstract language when the situation or purpose calls for it. Keats, though he was one of the most concrete of poets, began Endymion with a sentence composed of abstract terms:
A thing of beauty is a joy forever: Its lovliness increases; it will never Pass into nothingness...
And some of the most moving and memorable passages in poetry are not concrete; for example, the statement about God in Dante's Paradiso 'In His will is our peace', or the bleak comment by Edgar in the last act of King Lear
Men must endure Their going hence, even as their coming hither; Ripeness is all
A Glossary of Literary Terms - M. H. Abrams