An impersonal clause construction is a construction that lacks a °subject argument or whose °subject argument is an °impersonal-argument form.
"The term ‘impersonal’, as used in the linguistic literature, has received both a structural and a communicative-functional characterization. From the structural point of view impersonalization is associated with the lack of a canonical subject, from the functional perspective with agent defocusing." (Siewierska 2008: 116)