i think that the the phrase "long term relationship" doesnt necessarily depend on the amount of time you have been with your partner, but instead how commited you are, and if you are planning to be still with the same person for the "long term". Obviously this kind of relationship usually grows over time, you don't instantly see yourself with that person for the rest of your life. I think this is why people sometimes confuse a "long term relationship" with a "long" relationship. Some people stay with a partner for a few years but never intend on spending their whole lives with them, or setting up a family with that person.
I think defining a relationship as "short term" just means that you see yourself as a kind of 'here and now' relationship, thinking about the present primarily, and not about spending your life with that person.
But thats only my opinion xxx