x adjacent to y if and only if x and y share a boundary.
This relation acts as a join point with BSPO
A caterpillar walking on the surface of a leaf is adjacent_to the leaf, if one of the caterpillar appendages is touching the leaf. In contrast, a butterfly flying close to a flower is not considered adjacent, unless there are any touching parts.
The plasma membrane of a cell is adjacent to the cytoplasm, and also to the cell lumen which the cytoplasm occupies.
The epidermis layer of a vertebrate is adjacent to the dermis.
The skin of the forelimb is adjacent to the skin of the torso if these are considered anatomical subdivisions with a defined border. Otherwise a relation such as continuous_with would be used.