Here is a basic definition of synthesis:
The use of outside sources to gather information and opinions, in order to develop ideas, amass evidence, and support arguments. Synthesis enables writers to do more than simply express their opinion—it enables them to enter the conversation about their topic already being held in the wider world. It also allows them to complicate their ideas, to see more than one side, and to marshal information and logical arguments in the service of their position.
Cohen, Samuel, editor. 50 Essays: A Portable Anthology. 4th ed., Bedford, 2014. p. 490.