These words refer to something looked at as an entity in isolation from the whole of which it is a member. Part is the most general of these, referring to a quantity, sometimes amorphous or unspecified, of a particular whole: asking for a part of his sandwich; in one part of the book I’m reading. It can refer also to some independently structured member of a totality: the spare parts needed to repair his car.

Piece and portion relate exclusively to that aspect of part suggesting a quantity drawn from a whole. Piece is especially appropriate in reference to flat stretches of material, such as land or cloth: a piece of property; piece of calico. The word can also be used for other quantities: a piece to abstract qualities: a cafeteria that serves lavish portions ; a man who possesses enviable portions of wisdom and tolerance. Portion , unlike piece , also has a specific use in reference to a quantity of time: spending a portion of each summer at the seaside. Passage is more informal than portion and is even more commonly used to refer to periods of time, especially to elapsed time: a passage of ten years during which I did very little travelling. It also has a special use in reference to parts of art works, such as literature and music: a passage from the Second Symphony that echoed in his mind whenever he thought of Paris; the passage in his book in which he comes out most strongly for freedom of the press.

The remaining words all refer to clearly defined entities or groupings within a larger, structured unity. Subdivision is the most specific of these, suggesting a rigid logical organization of part to part within a complex framework, such as a bureaucracy, a schematic drawing of surveyed land, a legal document with numbered or lettered paragraphs, or any detailed outline or system of classification: such catalogued subdivisions of non-fiction as biography, history and travel. Segment can refer to a specified part of a geometrical figure or to some natural subdivision : Arcs are segments of circles. It can also be used as a more formal word for a piece of some linear quantity, such as thread or wire: laying a new segment of cable to connect the two stations. Section is the least specific of these three words; it is more closely restricted than segment to refer to a structured part of an organized entity, but suggests less complexity than subdivision : that section of the Department of Civil Aviation dealing with ground facilities; a wealthy section of town. When it approaches part in generality, it suggests a definable slice of a whole, sometimes isolated for the convenience of close inspection: calling his attention to a section of the portrait in which a later over-painting was clearly noticeable; the section of her will in which she disposed of her household effects.