How is an emotion different from a thought or a sensation? I don't know any good definition of emotion. I don't know what they really are, subjectively. 
I don't understand what makes an experience an emotion, as opposed to a thought or a sensation.


What is a good definition of emotion?
How is an emotion different from a thought or a sensation?
Right. No need to reinvent the wheel, so let me Google that for you...

Emotion


  a natural instinctive state of mind deriving from one's circumstances, mood, or relationships with others.
  "she was attempting to control her emotions"
  synonyms: feeling, sentiment; reaction, response
  $\quad\quad\quad\quad$"she was good at hiding her emotions"
  $\quad\quad\quad\ $ &middot; passion, strength of feeling, warmth of feeling
  $\quad\quad\quad\quad$"overcome by emotion, she turned away"
  
  &middot; instinctive or intuitive feeling as distinguished from reasoning or knowledge.
  $\ \ $"responses have to be based on historical insight, not simply on emotion"
  $\ \ $synonyms:   instinct, intuition, gut feeling; sentiment, the heart
  $\quad\quad\quad\quad$"responses based purely on emotion"