Introduction to Lie Algebras and Representation Theory
《Introduction to Lie Algebras and Representation Theory》 · 印刷第 94—95 页 Section 17.5「Free Lie algebras」，引入术语的末句及跨页定义
For this one needs the notion of free Lie algebra. Let $L$ be a Lie algebra over $F$ generated by a set $X$. We say $L$ is free on $X$ if, given any mapping $\phi$ of $X$ into a Lie algebra $M$, there exists a unique homomorphism $\psi:L\to M$ extending $\phi$.