Most typically the description used for dialup modem connections over copper wire based telephone (POTS) networks up to 56Kbps. In the UK it also references any technology with downstream speeds below 128Kilobits/per second (Kbps).
Related Articles:
> Dialup
> Modem
> POTS - Plain Old Telephone Service
> PSTN - Public Switched Telephone Network
> Kbps - Kilobits Per Second