Tperf is a term used by Teradata Engineering when sizing systems for a customer. It is a relative performance number. They arrive at it by running a series of queries that they feel represent an "average" workload. I don't believe that the queries are published, so I think that they're only available internally. It is meant to be able to compare the performance of two different configurations.
For instance, if one machine has a Tperf of 30 and another has a Tperf of 60, the 2nd one can get roughly twice as much done in the same amount of time or will get the same amount of work done in half the time.
It is very useful when ordering new hardware to replace old hardware or for ordering new hardware to be used in a co-existence system.