expressivity
If penetrance asks whether a light comes on, expressivity asks how brightly. Among people who do express a genotype, expressivity measures the degree or severity of that expression — from barely noticeable to dramatic — even though their underlying genotype is the same.
When the severity differs from person to person, we say the trait shows variable expressivity. One family member might have only a faint version of a condition while a relative with the identical allele has a severe form, and both are correctly described as affected.
The variation comes from factors layered on top of the main gene: other modifier genes, the environment, chance events during development, and in some traits the particular mutation involved. Expressivity and penetrance are distinct knobs — a trait can be fully penetrant (everyone with the genotype shows something) yet highly variable in expressivity (how much they show).
Neurofibromatosis type 1 shows striking variable expressivity: within one family, the same mutation can cause only a few skin spots in one person and many tumors in another.
Same allele, very different severity — the meaning of variable expressivity.
Penetrance is yes-or-no across people; expressivity is how-much among those who are affected. The two often vary independently.