Define load factor

Load factor $\lambda$ is the ratio of the collapse load (the load that produces a plastic collapse mechanism) to the working (service) load:

$$\lambda = \frac{W_c}{W_w}$$

In plastic design the working loads are multiplied by this factor (specified by the code, which gives different values for dead, live and wind combinations) and the structure is designed to just collapse under the factored loads. For a section it is related to the shape factor: $\lambda = S\times F$, where $F$ is the factor of safety against first yield (when moments are proportional to the loads).