There are a couple of ways to do this. The first is simply use the node GET call and pass in the qname like this:
GET /repositories/{repositoryId}/branches/{branchId}/nodes/{qname}
http://api.cloudcms.com/docs#!/node/get_repositories_repositoryId_branches_branchId_nodes_nodeId
This will give you back the JSON for the definition.
Another way is to use this variation:
GET /repositories/{repositoryId}/branches/{branchId}/definitions/{qname}
http://api.cloudcms.com/docs#!/branch/get_repositories_repositoryId_branches_branchId_definitions_qname
This will give you back the same JSON but may a few additional fields such as "_capabilities" (which describe all of the capabilities that the current user has against instances of that type) and/or "_forms". You can also get forms for a definition like this:
GET /repositories/{repositoryId}/branches/{branchId}/definitions/{qname}/forms
http://api.cloudcms.com/docs#!/node/get_repositories_repositoryId_branches_branchId_definitions_qname_forms
For any of these methods, the "qname" is the type of your definition - such as "my:article".