Dissatisfied with the definitions of Semi-structured data I've found, I decided to roll for dolo:
Semi-structured data are data that are primarily not suitable for representation with the relational model. Often such data are better expressed by less restrictive forms of propositional logic which might be as simple (and open-ended) as a hieararchical model (XML for instance) or as complex as 'proper' First-order Logic.
via Copia