The story
From tacho- + -cline; coined in a paper by Edward A. Spiegel and Jean-Paul Zahn in 1992 (Spiegel, E. A.; & Zahn, J.-P.; "The solar tachocline", Astronomy and Astrophysics, 265, 106–114, 1992) by analogy to the oceanic thermocline.
Etymology from Wiktionary, CC BY-SA 4.0.