What does it mean when a machine learning algorithm is referred to as a "black box"?

Select an answer:

A black box is a decoy process that secretly channels processing to other, standard algorithms.

A black box is a private, proprietary mechanism that is not viewable by the public but that is viewable to its owners.

A black box means a process that is hidden from view or difficult to understand.

A black box is a process that paradoxically produces accurate output even though the process is known to be flawed or incomplete.