You guys know the kanji: 合ってる.
It seems like the word is being treated as a noun, like "ki" or "kokyu". But actually it is a verb, in a progressive conjugation.
合う - au "to match, to meet"
合って（い）る - atte-(i)ru "matching, meeting"
It seems like Endo-sensei was using it particularly idiomatically to indicate a certain kind of matching, perhaps an interaction with the opponent's ki, the ki "matching".
This reminds me of something I heard and saw demonstrated by an Iwama instructor in Nagoya (Takumo Sensei of Saito Hirohito's Iwama Shinshin Aiki Shurenkai). He told a student that he was doing great, but he would really improve when he grasped "awase".
Now, "awase" is written with the same kanji as "aiki", and the same kanji as "atteru": 合わせ. But while 合う is intransitive, 合わせ is transitive (in this case in a gerund form). The instructor went on to explain "awase" as (he said) Saito Morihiro used to demonstrate it.
He had the student grab his hand in morotetori, as hard as he could (as we are wont in Iwama style). With the student holding on hard, he said, "With awase, you should be able to scratch your head, and your butt." Then with a slight movement, vaguely similar to morotetori kokyuho, he reached up and scratched his head, despite the students efforts to hold his arm down. Then he brought his hand down on the same track, slightly turned his body, and scratched his rear-end. There was no straining, or muscling up; he moved his hand up as if the student wasn't there, and then back down the same way.
Working out with this exercise, it definitely reminded me of my work out with Rob John. I don't actually think Takumo-sensei is a master of internal power (though I may be wrong!), but I think what he was teaching was certainly a fundamental aspect of aikido, and if nothing else the very beginnings of internal training.
Essentially, IMO, Endo-sensei and Takumo-sensei were talking about the "ai" of aikido, what exactly it means.