designating or involving an equation whose terms are not of the first degree
"The physicist struggled to solve the nonlinear differential equation because its terms were not all of the first degree."
Not lying on a straight line.
"The growth curve of the virus population is nonlinear because it does not follow a straight path over time."
In plain English: Nonlinear means something that does not follow a straight line or happen at a steady, predictable rate.
"The relationship between how much sugar I eat and my energy levels is nonlinear, because eating too little leaves me tired while eating a huge amount crashes me later."
The word nonlinear is formed by combining the prefix non-, meaning not, with linear to describe something that does not follow a straight line or direct proportion. It entered English as a straightforward descriptive term for mathematical concepts and phenomena that deviate from simple linear relationships.