Systemic describes what relates to or affects an entire system. For example, a systemic disease affects the entire body or organism, and systemic changes to an organization have an impact on the entire organization, including its most basic operations.

adjective of or relating to a system, especially when affecting the entirety of a thing: systemic flaws in the design and construction of the vehicles.

relating to or involving a whole system: The problems are systemic and will only worsen.

Systemic is somewhat more formal, and it is primarily used to describe what relates to an entire system, be that system physical, organizational, or societal. A systemic disease affects the entire body or organism; systemic changes to an organization impact the entire organization; and a systemic problem in a society pervades the entire society.