Depending on the context it could be translated as: estate, settlement, neighborhood. Usually, 'osiedle' refers to a housing estate which is a group of homes and other buildings built together as a single development. In cities 'osiedla' usually have names like Osiedle Stefana Batorego in Poznań. It comes from the verb 'osiedlać/zasiedlać' (to settle). It's related to 'siedlisko' (dwelling, habitat, abode) coming from the verb 'siedzieć' (to sit). Colloquially we tend to say 'siedzę w domu' (I am sitting at home) when we mean 'I am at home'.

Its' like district but smaller. Neighborhood.