grade

grade
1. Group of things all of which have the same value.

2. A balanced condition, especially of a river (river or stream grade) when it has just sufficient energy to transport the load supplied from the drainage basin; a balance between erosion and deposition. The concept has also been applied to hillslopes (‘graded slopes’) that are stable dynamically and so maintain themselves in the most economical configuration. The term is no longer used widely as it oversimplifies the issues involved.

3. The fraction of a sediment falling within a particular size limit, e.g. sand grade, silt grade, and boulder grade. See PARTICLE SIZE.

4. The quality of a mineral ore.

5. Classification of an ore by the quantity or purity of the mineable metal in an orebody.

6. In civil engineering, the gradient of a road.

7. Distinctive functional or structural level of complexity in the organization of an organism. Thus fish, amphibians, reptiles, and mammals represent successive vertebrate grades. Grades may occur within a single lineage; or the same grade may be achieved independently in different ones (e.g. warm-bloodedness evolved independently in birds and mammals).

8. See METAMORPHIC GRADE.