Extremely or immeasurably small.
Example:
Looking more closely at the research data, he now saw an odd pattern of changes so infinitesimal that they hadn't been noticed before.
Infinitesimal includes the negative prefix in-, "not"; the resulting word describes something endlessly small. When Antonie van Leeuwenhoek invented the microscope in the 17th century, he was able to see organisms that had been thought too infinitesimally small to exist. But today's electron microscope allows us to see infinitesimal aspects of matter even he could not have imagined.