There is tangential speech and circumstantial speech. They are not uniquely characteristic of bipolar, though - people who suffer from schizophrenia may speak like that.

Also, although linear speech pattern is considered the norm, sometimes topics are too complex not to follow tangential paths.

I think that provided you can return to the main idea, can understand that you digressed, then it is not characteristic of thought disorder.

Circumstantial and tangential speech is NOT word salad!