The BT x86 assembly language instruction stands for Bit Test and was added to the x86 instruction set with the 80386 processor. BT copies a bit from a given register to the carry flag.
Example: copy the third least significant bit from EAX to the carry flag
BTS (Bit Test and Set) operates the same, but also sets the bit in the register, while BTR (Bit Test and Reset) resets it, and BTC (Bit Test and Complement) flips it.
References
Bit Test Wikipedia(Text) CC BY-SA