What is spontaneous generation?
The theory that microbes appear from nothing
This theory was used to explain how microbes are found to grow overtime (eg mould) and where they came from. This theory was eventually rejected by Pasteur due to his famous Swan Neck Flask experiment, where nutrient rich broth was contained in a flask with a long swan neck-like tube at it's rim.
After a long period of time, the broth did not become contaminated. Upon removing the neck, the broth quickly became contaminated. Pasteur concluded that microbes do not 'spontaneously generate' but rather they exist throughout the environment and multiply if/when they find themselves in favorable conditions.