(By Anna Siewierska): "A construction has been classified as passive if it displays the following five properties: (i) it contrasts with another constuction, the active; (ii) the subject of the active corresponds to a non-obligatory oblique phrase of the passive or is not overtly expressed; (iii) the subject of the passive, if there is one, corresponds to the direct object of the active; (iv) the construction is pragmatically restricted relative to the active; (v) the construction displays some special morphological marking of the verb. A prototypical example of the passive, as defined above, is given in (1b) from Swahili..."
As in the Grammaticon, Siewierska's definition of "passive construction" includes the requirement of verbal marking. So-called "unmarked passives" do not fall under the definition.