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 epidermis layer of a vertebrate is adjacent to the dermis.
The plasma membrane of a cell is adjacent to the cytoplasm, and also to the cell lumen which the cytoplasm occupies.
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.