ONT
acronymISP & telecom
Optical Network Terminal: the customer-side box that converts the PON's light into Ethernet - the fiber modem, in everyday speech.
ONU is the near-synonym from the standards; field techs use the terms interchangeably.
The ONT is the subscriber end of a fibre line, converting light to electrical signalling and presenting Ethernet to the customer. Where the operator terminates and the customer's own equipment begins is a boundary worth being precise about during a fault, because it decides whose problem it is.
The number that matters is received optical power. A reading within the expected range means the fibre path is sound and the problem is above it; a low reading means loss in the path, from a bend, a dirty connector, a damaged drop or a splitter issue. That single measurement resolves most access-network disputes before anyone is dispatched, which is why remote read of ONT optical levels is one of the higher-value capabilities an operator can have.