Take MIPS instruction format described here, there are some abbreviations eg rd,rs and rt. What does rt stands for? I know rt is the second source register in R-type instruction, and is the destination register in I-type instruction. It is easy to guess rd stands for register destination; rs stands for register source. What about rt? I guess it might be "register temporary"? Is there an authoritative source about this?
t is just the next letter after s, so rs is the first source register, rt is the second source register. (If there could be a third source register it would most likely be ru.)