Definition
An SMS segment is one 140-octet transport unit of a text message. A message longer than one segment is split into several, and each segment is usually billed as a separate SMS.
The 140-octet limit comes from 3GPP TS 23.040. It holds 160 GSM-7 characters, 70 UCS-2 characters or 140 bytes of 8-bit data.
Once a message exceeds a single segment, every part carries a user data header for reassembly, leaving 153 GSM-7 or 67 UCS-2 characters per segment.
Pricing is almost always per segment, so a 161-character GSM-7 message costs two SMS, and a 71-character Arabic message costs two as well.
To control cost, keep encoding in check (avoid accidental Unicode), shorten links, and count segments rather than characters before sending a large campaign.
Example
300 GSM-7 characters → 2 segments (153 + 147). 300 UCS-2 characters → 5 segments (67 × 4 + 32).