Party wall?

My property has a wall that runs alongside the street pavement and it is shared with the property behind, as in it is one structure. Roughly two-thirds of it is on my property and one-third is on theirs. My part is only used by my property and their part is only used by them. There's nothing in my legal paperwork regarding this wall and nothing marked on the deeds. Is this a party wall? Sorry if this is a stupid question.