A companionship is a relationship. Every interaction we have with another human being is a result of our relationship to them. That said, I believe companionship is just a deeper, more consistent and long lasting bond that happens between two people. It may not necessarily be romantic, but it can be. Companionship (to me) seems to indicate an ease of existing together in the same place even if you partake in different activities. It's being together in a physical sense and also confiding in and supporting each other in an emotional sense. I think that where it takes a departure from a romantic relationship is that romance tends to be about becoming a unity and merging lives together based on an envisioned future together. I think there is an element of companionship that comes along with a long term romantic relationship but one does not always imply the other.