I've often wondered what most people considered a plateau also.
I did the same thing this past month, added exercise and "plateaued" as far as I was concerned because the scale didn't budge up or down. It's finally moving again so I'm a little more upbeat about it now.
It took a good full month for things to move again, but I am so much stronger, have much more endurance and my BP is almost normal!!
122/70 last night.
I now track weight, waist and BP to keep me motivated. As long as my BP is going in the right direction, who could possibly complain, ya know?