A memory bank is any device intended for holding information over an extended period. While the term implies only computer-based devices, it technically includes hard copies, cave drawings, and others. Physical sizes of memory banks range from a hand-held data card measuring as little as a half-inch, to massive constructs the size of buildings, with data capacity ranging accordingly.
The memory banks of the alien probe Tan Ru were transferred and merged with those remaining in the Earth probe Nomad when the two spacecraft met in deep space. This created a probe with the unintended directive to "seek out and sterilize new life," with Nomad as the body and predominant voice during their co-existence. (TOS: "The Changeling")