Bi-polar/medical labels

Is a diagnosis such as bipolar a circumstance or a thought? For example, my friend told me she has been diagnosed with bipolar 1, but I’m kind of confused about the diagnosis. Isn’t a diagnosis just a doctor’s thoughts/interpretation? I’m a little confused about mental medical labels in general as to what do they even mean, and how to know if the label themselves isn’t just a thought? Any help is so greatly appreciated!! 🙂