This paper gives a summary of previously published papers on the definition of autonomy for ships, how this relates to different crewing regimes, and the terminology to be used. A conclusion is that autonomy should be retained as a descriptive term, but that we should distinguish between "full autonomy" and "constrained autonomy", where the latter is the more relevant term for ships today. The proposed classification of autonomy is related to both degree of automation and degree of human control and will be presented as a matrix with generic classes of autonomy. This matrix is also transformed to a set of more practically useful levels of autonomy based on likely organization of crew on land or on the ship. The paper has mainly been written based on our work with maritime autonomous surface ships (MASS) but is also applicable to other types of surface vessels, e.g. inland waterway vessels.