Taichi and Yamato having a romantic relationship. Most of the time Taichi is the dominant one. Yamachi is when Yamato is the dominant one but both can be used to just describe Taichi and Yamato having a relationship.
It totally rawks!! If you don't like it you suck!
person1: "OMGOMGOMGOMFG!! Taito fucking rocks!!"
person2: "Oh not another yaoi fangirl...."