Plain English Meaning
The oil that actually pours out when you open the drain plug. It is only part of the total oil in the engine — the rest stays inside coating parts and sitting in lines and the cooler.
Full Definition
The portion of engine oil that can be removed from an aircraft engine and its oil system through the normal drain points during a routine oil change. It does not include the oil that remains trapped in passages, galleries, the oil cooler, filter, and engine internals after draining.
Context Anchor
Seen in weight-and-balance discussions, especially when determining what is included in an aircraft’s empty weight.
Derivation
From 'drain' (Old English 'dreahnian,' to draw off liquid) plus the suffix '-able' meaning 'capable of being.' Together: the oil capable of being drawn off. The term exists because not all the oil in an engine can be drawn off — so a specific word is needed for the part that can.