Actually I am a little bit confused about the definition. I have read two three articles but I could not find out what type of equations are called a non-linear partial differential equation. Articles are following.
https://en.wikiversity.org/wiki/Partial_differential_equations https://www.slideshare.net/jayanshugundaniya9/advanced-engineering-mathematics-first-order-nonlinear-partial-differential-equation-its-applications https://mat.iitm.ac.in/home/sryedida/public_html/caimna/pde/forth/forth.html
$pq = 0$ will be a first order non linear Partial differential equation? p,q are usual notation in PDE.
Please don' downvote. I know it is a silly question. But I am really confused. Please help me. I am looking forward to ur reply.