Lake Acuity

Lake Acuity in the anime

Lake Acuity during the winter

The Lake Acuity was first featured in Pruning a Passel of Pals!, when it was mentioned by May and Zoey as the location where one of the lake guardians is known to live.

Ash and his friends were going to the Lake Acuity in Frozen on Their Tracks!, but due to Team Galactic's actions, the train was stopped at a fixed signal until the end of the episode.

The lake was the main setting of a Full Battle between Ash and Paul, which took place from Pedal to the Mettle! to Evolving Strategies!. The battle was refereed by Olivier, who was called by Reggie to preside over the match. Despite his efforts and the Evolution of his Chimchar into Monferno, Ash ended up losing to his rival.

After the battle, Ash and his friends stayed at Lake Acuity's Pokémon Center so that Nurse Joy could take care of Ash's Pokémon. In Uncrushing Defeat!, a silhouette of Uxie appeared at the lake and showed itself to Brock, who was trying to find Berries to help Ash's Monferno.

In The Needs of the Three!, it was explained that Lake Acuity, and indeed all three of Sinnoh's lakes, are the entrances to another world where the lake guardians reside, although they do not actually live in the lakes themselves.

During the Team Galactic finale, Professor Carolina was stationed here, helping Professor Rowan with trying to keep Uxie safe.

Lake Acuity in Pokémon Evolutions

Lake Acuity appeared in a flashback in The Rival. Barry and his Prinplup fought against Jupiter and her Skuntank at the lake, but wound up getting defeated, allowing Team Galactic to get away with Uxie.

In Hurrah for Rapidash, Platinum parted ways with Diamond and Pearl and headed to Lake Acuity in order to protect Uxie from Team Galactic. Accompanied by Maylene and Candice, she arrived there in Stopping Sableye. Upon arriving, Platinum recalled how her father had told her that she had been to Lake Acuity before, despite her not remembering anything about it.

The three were almost immediately confronted by Commander Jupiter, who was easily able to defeat them, despite two of them being Gym Leaders. After the Galactic Bomb was detonated at Lake Valor in Tackling Tangrowth, Uxie emerged from the Acuity Cavern and was quickly captured by Team Galactic.

Lake Acuity in Pokémon Diamond and Pearl Adventure!

Lake Acuity appeared in The Battle Rages On and A Novel Test!!, where Mitsumi battled and defeated Jupiter there. However, by bringing up Mitsumi's past as a Team Galactic member, Jupiter was able to throw off Mitsumi and shock Jun, allowing her to take the latter hostage. She then contacted Cyrus, who ordered her to bring Mitsumi to the Team Galactic HQ. Mitsumi refused at first, but Cyrus was able to change her mind, and she reluctantly left with Jupiter, leaving Jun behind in the snow, where he was later found by Hareta.

Acuity means "sharpness of wit". This is fitting, as Uxie is the Knowledge Pokémon.

Eichi is from 叡智 eichi (wisdom, intellect), as indicated by a signpost in Brilliant Diamond and Shining Pearl that provided an annotation of its name fully written in kanji as 叡智湖.