SIMT - Single Instruction, Multiple Threads

Threads.

It's less a difference in instruction set capability and more a difference in mentality.

Like, for SIMD, you have to say "ok, we're working in vector land now" and start doing vector loads into vector registers to do vector ops on them. Otherwise, the standard variables your program uses are scalars and you get less parallelism. On a GPU this is flipped: the regular registers are vector, and the scalar ones (if you have any) are the weird ones. And because of this the code you write is (more or less) scalar code where everything so happens to magically get done sixteen times at once.

As you can imagine, this isn't foolproof and there's a lot of other things that have to change on GPU programming in order to be viable. Like, conditional branching has to be scalar, since the instruction pointer register is still scalar. But you can have vectors of condition flags (aka "predicates"), and make all the operations take a predicate register to tell which specific lanes should and shouldn't execute. Any scalar conditional can be compiled into predicates, so long as you're OK with having to chew through all instructions on both branches[0].

[0] A sufficiently smart shader compiler could check if the predicate is all-false or all-true and do a scalar jump over the instructions that won't execute. Whether or not that's a good idea is another question.