It's the load at which the structure is in a state of incipient plastic collapse.
Beyond this, the load-displacement response becomes non-linear and the plastic or irreversible part of the displacement increases steadily with the applied load.
Engineers use limit states to define and check a structure's performance.
Bounding Theorems of Plastic-Limit Load Analysis: Plastic limit theorems provide a way to calculate limit loads without having to solve the boundary value problem in continuum mechanics.
Finite element analysis provides an alternative way to estimate limit loads.