interfaces

interfaces: {
    <InterfaceTypeName>: {
        description: <description>
        fields: {
            <fieldName>: {
               type: <graphQLType>,
               args: {
                 <argName>: <graphQLType>,
                 // the rest of arguments ...
               }
            },
            // the rest of field definitions ...
        },
    },
    // the rest of interface definitions ...
}

interfaces: {
    CustomInterface: {
        description: "Custom Interface",
        fields: {
            query: {
                type: graphQL.list(graphQL.GraphQLString),
                args: {
                    filter: graphQL.reference('CustomFilterInput')
                }
            }
        },
    }
}