A T5 chip is similar to a $5 chip, but has no currency symbol -- because it has no cash value, and is typically used in tournaments. Instead of five dollars, a T5 chip represents five units.
A T5-base set (or tournament) is one where the smallest denomination is the T5 chip.
The starting stack in a tournament can also use the T reference. A typical starting stack for a T25-base tournament is:
12 x T25
12 x T100
5 x T500
6 x T1000
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35 chips, totaling T10000 tournament units.