The original low cost SDR was a European TV tuner USB stick. A driver developer noticed that it was possible to turn off the built-in vertical and horizontal blank suppression to get a raw I/Q dump from a device that was available for $20 retail. This revolutionized the hobbyist SDR community as the purpose built devices cost hundreds or thousands of dollars.

The "RTL" came from the company that built the hackable chip: Realtek.

I believe the RTL in RTL-SDR is "Realtek Limited", the manufacturer of the chips used in the early days of SDR. I don't think the chips these days are exclusively Realtek, but the name has persisted.