The story
First attested 1902 (OED) in the electrical engineering sense "the resetting of a coherer." From decohere (also 1902) + -ence, representing the verbal noun (compare decohesion, decoherency). The quantum mechanics concept was introduced in 1970 by German physicist H. Dieter Zeh, writing in the journal Foundations of Physics.
Etymology from Wiktionary, CC BY-SA 4.0.