Also Resistor-Transistor Logic, an older generation logic family.

RTL is a family of private TV stations in Germany (though there are also RTLs in Luxemburg (RTL does stand for something like Radio and Television Luxemburg) and the Netherlands, but I do not know much about them).
In Germany there are three RTLs:
RTL - the main station, is one of the most popular tv stations in Germany, has new films, talkshows and sport
RTL II = started as a station to send all the old RTL material, now it is well known for children programmes, big brother, soft-sex films and reports
Super RTL - is now what RTL II has been in the beginning, old cartoons, old films, old series and really bad music shows.

In digital design, RTL means Register Transfer Level. RTL is the lowest design level, where you design the actual registers (flip-flops) and the data flows between them.