LW is the written short form of long wave, a kind of radio. On an old radio, you may see LW, MW, SW, and FM as choices. Long wave uses radio waves that are, in simple terms, “long”: they can travel a long way, especially over land at night, which is why some countries used LW for news and talk that had to reach far.

You mainly meet LW as letters on a dial, in a manual, or in a list of stations — it is a written abbreviation. People often say “long wave” out loud rather than “L-W.” Though actually, LW can mean other things in other jobs (a size, a code, a science label), but on a radio it means this. It is not the same as FM, which is usually clearer but does not reach as far in the same way, and it is not a word you use in ordinary chat unless you are talking about radios.