Final answer:
Louis Pasteur's idea that life does not spontaneously arise but is introduced via airborne microbes is classified as his Hypothesis. This hypothesis was central to his refutation of the theory of spontaneous generation.
Step-by-step explanation:
Louis Pasteur's belief that airborne microbes were the cause of life appearing in meat broth is considered his Hypothesis. Pasteur's swan-neck flask experiments were pivotal in disproving the theory of spontaneous generation. Rather than accepting the notion that life can arise from non-living matter spontaneously, Pasteur hypothesized that life is introduced to these environments through tiny organisms present in the air. The essence of Pasteur's work was captured by his phrase "Omne vivum ex vivo" (“Life only comes from life”), a direct contradiction to the spontaneous generation theory. His hypothesis was then tested through meticulous experimentation that led to the conclusion that the presence of microorganisms in previously sterile meat broth was due to their entry from the environment, not from an inherent 'life force' within the broth itself.