the.com/boundary condition

means A boundary condition is an extra fact about the edges or starting state of a system that you need, alongside the main equation, to pin down one specific solution out of all the mathematically possible ones.

from The phrase comes out of 19th-century mathematical physics, where differential equations describing things like heat flow, vibrating strings, or electric fields were found to have whole families of valid solutions — the equation alone couldn't choose. 'Boundary' refers literally to the edges of the physical region in question (the ends of the string, the surface of the object), and the 'condition' is the known value or behavior there (fixed temperature, zero displacement, and so on). Specifying those edge facts is what lets you select the one solution that actually matches reality. The term stayed inside physics and engineering for a long time before drifting into looser, figurative use — a limit or constraint that shapes which outcomes are even possible.