Science is the systematic study of the natural world and how it works. It involves gathering and analyzing evidence to understand and explain phenomena [observable events or occurrences]. For example, scientists study the movement of planets in space [large objects like Earth or other celestial bodies that revolve around a star] or the reaction of chemicals to being heated, mixed, etc.
Very Simple Definition:
Science is the way we learn about things and figure out how they work. Scientists use experiments and observations to find answers to questions. For example, scientists might watch how a plant grows or test what happens when they mix different chemicals.