When I have a library of fonts, I can count the number of fonts, or the number of families. In such a case, yes, I do refer to those lone styles as each being a family. So does just about every font vendor and font management app, I think.
Those are both reasonable points, but I think Rémi is asking about conventional usage when describing what you've made (a font or a set of related fonts). If you announce the release of a new type family and it only has one member, people would think you were a little strange.
@Mark is on the right track, except it is a virtual family made up of at least two members of a real family. Sort of like imaginary friends they find comfort in talking too, perhaps?
Well, a family is a set, and a set is something containing two or more different things, right? So, only one font, even with its various formats, can not be a family, or am I wrong?
Also, I think saying that one font is a family may confuse customers, they may think "If it's a family where are the other styles?", and then they go away...
In the mathematical sense, a set can contain one or even zero members (empty set). But a type family is a not a set in the mathematical sense. It's a set in the colloquial sense, where a set is normally taken to mean that it contains two or more things.
A typeface can be represented by a single font, or a family of them. Also, talking about typefaces is a bit classier and more professional than saying “fonts”, because in lay talk, they say things like “The Helvetica font”.
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Also, I think saying that one font is a family may confuse customers, they may think "If it's a family where are the other styles?", and then they go away...
Also, talking about typefaces is a bit classier and more professional than saying “fonts”, because in lay talk, they say things like “The Helvetica font”.