There has been a resurgence of interest recently in the nature of solidarity. A problem, however, bedevils any attempt to defend any one view against another. What makes disagreement about the nature of solidarity meaningful? Why should we think any one usage ought to be preferred to any other? The questions I raise are about an important social phenomenon, and so are of significance to, among others, social and political philosophers. But they also raise difficult issues in what is now often referred to as conceptual ethics—about the standards we should use in deciding which concepts to use and when to use them. This article argues that we should conceive of solidarity as a distinctive type of social kind. Seen in this light, the discussion should contribute to understanding the nature of solidarity in at least three ways. First, it will help to allay skepticism that accounts of solidarity are really just accounts of the meaning of a word in a given language. Second, it will aid us in responding to the objection that solidarity is too vague or amorphous as a concept, and so can mean anything to anyone. Third, it will help us to identify an agenda for empirical and philosophical research on solidarity. The paper tries to make progress in two areas—research on solidarity and research on social kinds—by showing how one can inform the other.