A manufactured nutrient product with concentrated, immediately available nutrients, applied directly to soil or foliar spray, fast-acting but less soil-building than organic.
Synthetic fertilizers (like 10-10-10 from a bag) are chemical compounds providing fast nutrient availability. Plants use them immediately but they offer no soil-building benefit. They leach easily (nutrients wash away) and can cause nutrient burn if overapplied. They're cheaper and faster than organic fertilizers but don't improve soil health long-term.
Understanding synthetic versus organic helps you choose based on goals. Fast response or long-term soil building? Different products for different needs.