Definition of Accessio

In Ancient Roman property law, accessio is a term which signifies that two things are united in such a way that one is considered to become a component part of the other; one thing is considered the principal, and the other is considered to be an accession or addition to it. Sometimes it may be doubtful which is to be considered the principal thing and which the accession. But the owner of the principal thing, whichever it is, became the owner of the accession also.