A segment is the unit an SMS is billed in: one 140-byte message part, holding 160 GSM-7 or 70 Unicode characters when alone, 153 or 67 when part of a longer concatenated message.
Carriers charge per part transmitted, not per message as the user sees it. A 300-character GSM-7 text is two segments (153 + 147), a 300-character Arabic text is five (67 each), and both appear as one message on the phone. The rate card is therefore always a price per segment.
The arithmetic is worth doing before a send: a 161-character message costs twice a 160-character one. Most transactional texts fit in one segment by design; marketing texts are the ones that creep.
At SMSMeteor: every rate on the rate card is per segment; the API returns segments and price for each message, and campaigns are priced on the longest rendered row. Up to 6 parts per message.