Definitions of concrete:

• noun:   a strong hard building material composed of sand and gravel and cement and water
• verb:   form into a solid mass; coalesce
• verb:   cover with cement

  Example: "Concrete the walls"

• adjective:   formed by the coalescence of particles
• adjective:   capable of being perceived by the senses; not abstract or imaginary

  Example: "Concrete objects such as trees"