Abstract: In order to better understand the concept of modes, the characteristics of modes are analyzed and compared with those of complex modes or nonlinear modes. The connotation of a mode includes the synchronicity of the modal vibrations, the invariance to the initial conditions, the orthogonality among modes, and the superposition of the modal vibrations into the response. A complex mode concerns with the synchronicity but not the invariance, while the orthogonality and the superposition hold only in the state space. A nonlinear mode concerns both with the synchronicity and the invariance, but not the orthogonality nor the superposition.