A commutative ring $K$ is called a $\lambda$-ring if we are given a family of set operations $\lambda^k \colon K \to K$ for $k \geq 0$ such that for all $x,y \in K$

Note: there is a group homomorphism $\lambda_t$ from the additive group of $K$ to the multiplicative group $W(K) = 1+tK[[t]]$

A special $\lambda$-ring is a $\lambda$-ring $K$ such that the group homomorphism $\lambda_t \colon K \to W(K)$ is a $\lambda$-ring homomorphism.

References

Charles A. Weibel, The $K$-book: An introduction to algebraic $K$-theory, American Mathematical Society, Graduate Studies in Mathematics Volume 145, 2013.