From The Free On-line Dictionary of Computing (19 January 2023):
segmentation
reassembly
SAR
segmentation and reassembly
<networking> (Or "segmentation and reassembly", SAR) Breaking
an arbitrary size packet into smaller pieces at the
transmitter. This may be necessary because of restrictions in
the communications channel or to reduce latency. The pieces
are joined back together in the right order at the receiver
("reassembly"). Segmentation may be performed by a router
when routing a packet to a network with a smaller maximum
packet size.
The term "segmentation" is used in ATM, in TCP/IP, it is
called "fragmentation" an is performed at the IP layer
before the "fragments" are passed to the transport layer.
See for example ATM forum UNI 4.0 specification.
[Better reasons?]
(1999-06-14)