A few possible definitions of “low income”:
A. Bottom quarter income. Approximates eligibility for full Pell grant.
B. Bottom half income. Approximates eligibility for (not necessarily full) Pell grant.
C. Needs/gets financial aid at some colleges if admitted. Approximates bottom 96-97% income.
D. Anyone who has to consider cost when sending kids to college. Approximates bottom 99.5% income.
While most people outside of these forums would probably use definition A or B or something similar (e.g. eligibility for free or reduced price lunch in K-12), it is likely that many on these forums use a definition more like C or D.