"Thunderbird" is AMD's code name for intergrated L2 cache memory, so technically, even Durons are "Thunderbirds". The name got a lot of recognition when the OEM-only SlotA "thunderbird" core CPU's starting flowing through the grey market. SlotA "thunderbird" core CPU's were never released in retail form due to incompatibilities with the KX133 chipset, they only run RELIABLY on the AMD 751 "Irongate" chipset.
Here's the line-up:
[*]Athlon "Classic", SlotA, 512k L2 cache, max cache speed 350Mhz 500-1Ghz
[*]Athlon "Thunderbird" SlotA, 256k L2 cache, runs at core speed 700-1Ghz
[*]Athlon "Thunderbird" SocketA, 256k L2 cache, runs at core speed 700-1.2Ghz
[*]Duron SocketA, 64k L2 cache, runs at core speed 600-800Mhz