In short: Qubic began life as as an initialism-turned-acronym, QBC, which stands for quorum-based computation.
Wikipedia: „A quorum is the minimum number of votes that a distributed transaction has to obtain in order to be allowed to perform an operation in a distributed system. A quorum-based technique is implemented to enforce consistent operation in a distributed system.“
Specifically, Qubic is a protocol that specifies IOTA’s solution for quorum-based computations, including such constructs as oracle machines, outsourced computations, and smart contracts. Qubic provides general-purpose, cloud- or fog-based, permissionless, multiprocessing capabilities on the Tangle. In the long term, Qubic will allow people to leverage world-wide unused computing capacity for a myriad of computational needs, all while helping to secure the IOTA Tangle: an IOTA-based world supercomputer.
More generally, a qubic is a what we call a packaged quorum-based computation that occurs according to the Qubic protocol. Below are some examples of different types of qubics – while they are distinguished here for clarity, they are all nevertheless variations on a single, general-purpose concept: the quorum-based computation, or qubic.