Mnemonic memory circuit
From Memory Alpha, the free Star Trek reference
A mnemonic memory circuit was a piece of late 23rd century computer technology.
Trapped in Earth's past, Spock attempted to construct a mnemonic memory circuit so that he and James T. Kirk might view the recordings trapped within Spock's tricorder – recordings that would tell them how McCoy had changed history, erasing the Federation, and thereby how they might correct the problem.
Spock found it difficult to work with 20th century technology, viewing it as little better than stone knives and bearskins. Nonetheless, he was able to construct a circuit that briefly allowed the stranded officers to learn that Edith Keeler was the key – history's proper shape depended on whether she lived or died. (TOS: "The City on the Edge of Forever")
