The substrate is defined as the underlying layer that makes the existence and operation of systems possible. This paper establishes that systems do not arise independently, but depend on conditions that determine what can exist, how components can form, and what constraints govern their behavior. The substrate is presented as the source of possibility and constraint. It explains why not all structures can exist in all conditions, and why the same system configuration cannot behave identically across different substrates. The limits observed within systems are shown to originate from this underlying layer rather than from structure or interaction alone. The concept is introduced in a domain-independent manner and applies across physical, biological, cognitive, and computational systems. It provides a basis for understanding how system formation, stability, and behavior are bounded by the conditions from which they emerge. This paper defines the substrate as a foundational layer and does not extend into its origin, measurement, or methods of modification.