Here are some .gifs and images from the lightning attack.1
First, Yoda "catches" the lightning and, after a noticeable pause, sends it back to Dooku:
If you look closely, the bolts appear to come from his fingertips just as they do from Dooku's fingertips:
Dooku attacks again, and this time Yoda deflects the lightning like a lightsaber deflecting a blaster bolt (no pause after he "catches" it):
Finally, we see the lightning enter Yoda's hand and disappear (he "catches" it but does not return it). We see him for several seconds after the lightning disappears, yet there is no visual indication that the energy goes anywhere.
Based on the first case -- when he "catches" the lightning and then sends it back -- it makes no sense for him to have dissipated the energy externally only to have to re-create the energy in order to send it back. It makes more sense that he temporarily absorbed the energy within himself, and then directed it back to Dooku. The fact that the lightning bolts leave his fingertips is further evidence that Yoda absorbed the bolt temporarily.
In the third case he "catches" the lightning and does not send it back to Dooku -- whereas he temporarily absorbed the energy in the first case, here he permanently absorbs it. The light from the lightning bolt enters the palm of his hand and he closes his hand into a fist around it, so all the visual cues indicate that he absorbed the energy internally.
The third case fits the definition of absorption/tutaminis. Merriam-Webster includes the following definition of absorb (emphasis added):
4 a (1) : to receive without recoil or echo (2) : endure, sustain (3) : assume, bear
b : to transform (radiant energy) into a different form especially with a resulting rise in temperature
This seems to be as clear of a case as possible of Yoda absorbing the energy from the lightning into his body.
Of course, for all we know, Yoda converts the energy from the lightning into heat on the surface of his hand (which we obviously can't see), or he uses an invisible Force barrier of some kind, or something else to (invisibly) avoid absorbing the energy internally. The script doesn't even mention the lightning attack, so it's no help. The best evidence from the visual cues in the movie is that Yoda did indeed absorb the energy internally.
1Captured from this video, with the lightning attack starting at 3:22. Due to file size constraints the .gif images only cover the most relevants part of the lightning attack, and every second frame was removed from them.