Arbitrary lines we draw as humans between logical parts of a system we’re modelling. E.g. the apple is separate from the apple tree. The boundary is the interface between apple and apple tree. It doesn’t really exist. It is the negative space between our units of decomposition (apple and apple tree). With the apple example you can approximate a boundary to a region of a couple square cm, but other boundaries are more amorphous, like the boundary between beach and ocean or earth and space. Also note, the boundary changes over time (temporally) as the apple blossom turns into an apple which is then separated from the tree.