Grade
How abnormal the cancer cells look under the microscope, from grade 1 (close to normal, slow) to grade 3 or 4 (wildly abnormal, fast). Grade is about behaviour; stage is about extent.
Pathologists assign grade from how well the cells resemble their parent tissue (differentiation), how variable their nuclei are, and how many are dividing; the schemes are tumour-specific, such as the Nottingham score in breast, Gleason in prostate, and WHO grades 1 to 4 in brain tumours. High grade predicts faster growth and earlier spread but also, often, better response to chemotherapy, and grade feeds into decisions about whether to add chemotherapy after surgery. Grade should not be confused with the 'grade' of a side effect, which is a separate severity scale.