You have already seen encoding (T10) in Book 1 and Book 2, where data is transformed into a representation that hides internal structure — a hash hides the raw data, a compressed format hides the original bytes. Here the same pattern appears at code level. An abstraction encodes a capability: it presents a consistent interface that hides the complexity of implementation behind a stable surface. The shape is the same: what varies is concealed, what is stable is exposed.