The term “thread” is used instead of “yarn” when referring to the appearance rather than the textile product, e.g., warp thread, weft thread, upper thread, lower thread, sewing thread.
Source:
Eberle, Hannelore; Gonser, Elke; Hornberger, Marianne; et al. (2022), Fachwissen Bekleidung, 12. Auflage, Haan-Gruiten: Verlag Europa-Lehrmittel, S. 56