[duschek]

Dummy is on lead. The declarer's RHO thinks he hears declarer name a card from dummy to be played and follows suit, he thinks, before dummy touches any card. The other players agree that declarer did not name a card.

Is this a lead out of turn, which declarer can accept, or is it a play out of turn?

[blackshoe]

Law 44B said:

After the lead, each other player in turn plays a card, and the four cards so played constitute a trick

So there is no "trick" until a lead has been made. The lead is the first card played to a trick (see the Definitions). The first card played to this trick was by a player who was not on lead. So it's a lead out of turn.

If the laws addressed the question of what the player thought he was doing, it might be different, but as far as I can see, they don't, unless the player was misinformed by an opponent about whose lead it was, which did not happen here.