  A consistent Riccati expansion (CRE) is proposed for solving nonlinear
systems with the help of a Riccati equation. A system is defined to be CRE
solvable if it has a CRE. Various integrable systems are CRE solvable.
Furthermore, it is also revealed that many CRE solvable systems, including the
Korteweg de-Vries, Kadomtsev-Patviashvili, nonlinear Schr\"dinger and
sine-Gordon equations, possess a common determining equation which describes
interactions between a soliton and a cnoidal wave.
