What does the T stand for in AXI4-Stream's TDATA, TVALID, TREADY, etc.?
It would appear to mean 'Transfer'.

The specification does not explicitly state it but it clearly implies it on p1-2, the second page of the spec':

STREAM TERMS

The following stream terms are used in this specification:

TRANSFER  A single transfer of data across an AXI4-Stream interface.
          A single transfer is defined by a single TVALID, TREADY handshake.
          See Handshake process on page 2-3.


The definition of a transfer is that it uses a TVALID/TREADY handshake. So a Transfer is that 'T' being made Valid and accepted by Ready. According to that definition text.