Synthesis is the act of combining elements to form something new. If you describe your bedroom decor as a synthesis of vintage and punk, we'll know you mean a mixture of these two styles.
Synthesis can be either concrete or abstract. Scientists use the word to talk about what happens when chemicals combine — think of photosynthesis: the process by which plants synthesize light, carbon dioxide, and water to produce food. If you call a movie "Batman meets Titanic," then that's synthesis as well. Synthesis can also refer to the act of combining ideas.