“Indeterminacy” or “quantum indeterminacy” refers to the uncertainty of results when measuring certain pairs of properties of quantum particles. This uncertainty is due to the Heisenberg Uncertainty Principle (HUP). Thus, the HUP is sometimes also called the “Indeterminacy Principle.” Indeterminacy is the phenomenon that the precision of one measurement reduces the precision of the measurement of the other property. See the Heisenberg Uncertainty Principle.

Note that indeterminacy is different from the non-deterministic aspect of quantum mechanics: Many interpretations of quantum mechanics postulate true randomness. That is, the behavior of individual particles cannot be calculated from prior causes; only the behavior of collections of particles can be calculated, and then, approximately. The behavior of collections of quantum particles is calculated based on probabilities.

In contrast, classical physics, which applies to the everyday world of tables and chairs, is deterministic. That is, in classical physics, the behavior of objects can be calculated based on the application of forces. In classical physics, there is no true randomness.

To reiterate, indeterminacy is not the same as non-deterministic.