In radio, longwave refers to parts of the radio spectrum with relatively long wavelengths. The term is a historic one dating from the early 20th century, when the radio spectrum was considered to consist of long, medium and short wavelengths. Most modern radio systems and devices use wavelengths which would then have been considered 'ultra-short'.
In contemporary usage, the term longwave is not defined precisely and its meaning varies across the world. In the US, for example, the Longwave Club of America is interested in "frequencies below the AM broadcast band", i.e., all frequencies below 535 kHz. (Lower frequencies correspond to longer wavelengths.)
In Europe, Africa and large parts of Asia (ITU Region 1), where a range of frequencies between 148.5 and 283.5 kHz is used for AM broadcasting (in addition to the medium wave band), the term longwave usually refers specifically to this broadcasting band.
The Region 1 longwave broadcast band falls wholly within the low frequency band of the radio spectrum (30–300 kHz); broader definitions of longwave may extend below and/or above it.
Read more about Longwave: Broadcasting