It's a question I often ask myself, since I've been trying to create my own definition of companionship rather than use the confusing Romantic-Platonic model. It's dependent entirely on the particular person and what makes sense for the two of us as individuals. With some, the usual definition of purely platonic friendship is quite apt. With others, Companion definition relationship feelings and the desired level of contact is deeper and more affectionate than that, but still doesn't necessarily ask for a full-on "romantic relationship" as society would see it.
In any case, it's a fluid definition, based on what organically makes sense between me and the other person.
I don't really feel the need to demand a certain "fixed outcome" from Companion definition relationship. I guess it's just a sense of intellectual compatibility, basic common core values, and providing understanding and mutually enjoyable company to each other, in a way in which the world is less boring and lonely in the moments when we come together.
I'm cautious of conflating companionship with "romance" for a reason: Which is why I prefer strong platonic relationships with an element of affection instead. Or if there are romantic feelings in the picture, I'd rather they're not squeezed into the stereotypical fairytale ending rulebook.
For me companionship is one mode of love. I'm understanding whole our life as different phases of love. We love many living things. We Companion definition relationship love for ourselves. We love our parents, children, relatives, friends, partners, environment, work etc. Only there are different modes of love under that Loving Umbrella. Love can be plain or deep. Companionship is quite deep love. Are you making "companionship" your ideal kind of relationship, Companion definition relationship are trying to define that?
If that's the case I'm sure it'd be quite Companion definition relationship to you. Or are you just trying to define companionship as one way of relating to people and it's likely to be one factor of many?
If we are looking at it as one way of relating to people, I'd say it has a basis in friendship, with implications of deep trust and support and shared time together. To me it'd be important for that time together to consist of both shared interests and activities but also shared comfortable silences.
It's being able to be your authentic self and to be deeply bonded to someone else who is Companion definition relationship their authentic self.
That said, companionship is pretty close to what my ideal partnership would look like. Romance optional, sex optional.