2. Increase by something added; that which is added; augmentation from without; as, an accession of wealth or territory. The only accession which the roman empire received was the province of Britain. (Gibbon)
3. A mode of acquiring property, by which the owner of a corporeal substance which receives an addition by growth, or by labour, has a right to the part or thing added, or the improvement (provided the thing is not changed into a different species). Thus, the owner of a cow becomes the owner of her calf. The act by which one power becomes party to engagements already in force between other powers.