Final answer:
The buckling load is the axial load at which a column starts to deflect laterally and becomes unstable.
Step-by-step explanation:
The axial load at which a column begins to deflect laterally and becomes unstable is known as the buckling load. When a column is subject to a compressive force, it will buckle or deflect laterally if the applied load exceeds a critical value. This buckling load depends on various factors such as the column's length, material properties, and support conditions.