Addition, accretion, increment and accession all agree in denoting a thing that serves to increase another in size, amount, or content.
Addition implies union with something already existing as a whole or as a unit.
- he built an addition to his house last year
- the office boy, a recent addition to the staff, was busy with the copying press
—Archibald Marshall
Sometimes improvement rather than increase is stressed.
- the paintings were an addition to the room
Accretion implies attachment from the outside; it may be used of the process as well as of the thing added.
- a rolled snowball grows by accretion
It often suggests additions made to an original body over a considerable period of time.
- the professional historian, whose aim is exact truth, should brush aside the glittering accretions of fiction that have encrusted it
—Grandgent
Nearly always it implies the addition of unessential or alien matter.
- all progress in literary style lies in the heroic resolve to cast aside accretions and exuberances
—Ellis
Increment usually implies addition bit by bit in consecutive or serial order.
- the salaries are raised by annual increments
- one more wave in the endless ebb and flow of action and reaction, the infinitesimal increments of which we call Progress
—Lowes
Sometimes it signifies increase in value.
- benefited from an unearned increment in the value of his land resulting from growth of the city
Accession denotes something acquired that constitutes an addition to contents, holdings, or possessions.
- recent accessions to a library
- the greatest accession of positive knowledge has come in our own time
—Inge