I'd think this is one of the examples of a term being borrowed from other styles of budo in order to explain aikido. I believe that in several styles of budo there is a 'Uke nagashi' of some sort - some times with a different name. It can mean something a long the line of 'receive in flow'.
In the interpretation of Nishio sensei uke nagashi has become an aiki toho kata (https://www.youtube.com/watch?v=o3CCJaqx-_A
It has an equivalent in aikido techniques by performing aihanmi katatetori Ikkyo in a special way.. as shown here by Yoshida sensei: https://www.youtube.com/watch?v=0fSZNTstv2w
It is also being trained in paired situations with sword against sword (youtube ken tai ken uke nagashi) and jo against sword (youtube ken tai jo uke nagashi).
The principle as I see it is to receive the attack and - not deflect or counter - but slightly realign it by altering the focus of your center towards uke. This is - in my point of view - the most fundamental basic principle of all good aikido. Therefore I perceive it more as a principle than a technique - but with at strong relation in it's purest form to aihanmi katatetori ikkyo.
No doubt the term uke nagashi is known in many styles of sword, and since sword practice is one of the fundamentals for Aikido it has re-entered the terminology of our budo in various interpretations and meanings.
Just my thoughts on the matter
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