What does the T stand for in "T flip-flop"?
Some people believe that the "T" in "T flip-flop" stands for "toggle," but I believe that it originally was intended to stand for "trigger."
According to the Wikipedia entry on flip-flops,
According to P. L. Lindley, an engineer at the US Jet Propulsion Laboratory, the flip-flop types detailed below (SR, D, T, JK) were first discussed in a 1954 UCLA course on computer design by Montgomery Phister, and then appeared in his book Logical Design of Digital Computers. Lindley was at the time working at Hughes Aircraft under Eldred Nelson, who had coined the term JK for a flip-flop which changed states when both inputs were on (a logical "one"). The other names were coined by Phister.
In Montgomery Phister Jr.'s book Logical Design of Digital Computers (New York: John Wiley & Sons, Inc., 1958, pp. 126-127), he states
The T Flip-flop
The T or trigger memory element has a single input line which causes the memory element to change state when it is "one," but leaves it in its former state otherwise.
Because it seems that Phister was the first one to use the term "T flip-flop" and he used the "T" to stand for "trigger," it seems that this is what it was originally intended to mean.