It's something an object or a person could do or be used for. Example: the function of a clock is to tell time.
describe the function of the reproducive system
describe the notch and its function in the story
Photosynthesis
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Blood filter
No, x = 4 is not a function. I would describe x = 4 as the solution to an equation.
describe the structure of a fruit
True
describe the general business travel
describe alternative ways of organising the information systems function in a business
Describe the normal function of the hematic system