In chemistry, a synthesis reaction is a reaction in which multiple reactants combine to make one main product. However, when aspirin is produced, this is also considered a synthesis reaction even though more than one product is produced. I have attached a picture showing the reaction that takes place below. Can anyone explain why this is considered a synthesis even though more than one product is produced?
As was pointed out in the comments, it depends on your definition. The common usage of the word is taken to mean the series of steps (could be one, could be many) taken to make a certain compound. There is no specification in the definition that states that it MUST only have one major product (although this is usually the objective of synthetic chemists because we always want the targeted product with high selectivity).
It does not exclude the production of other "unwanted" chemical compounds which we call byproducts.
You could also look at it from the other angle, and ask what your definition of main is. In this case, it's the compound you want: aspirin. So there is only one main product, the others are byproducts.