**Update**.Suppose $\mathbf{B}$ is a category, $\mathbf{A}$ is a subcategory of $\mathbf{B}$, and $i:X\to G$ is an injective morphism. If for each object $Y$ of $\mathbf{A}$ and each morphism $f:X\to Y$ there is a unique morphism $\tilde{f}:G\to Y$ such that the following diagram is commutative:

\begin{xy}

\xymatrix{

X\ar[r]^f \ar[d]_i&Y\\

G\ar[ur]_{\tilde{f}} & ,

}

\end{xy}

then $G$ is said to be a *free* object with a *basis* $X$.

——

Suppose $\mathbf{A}$ is a category, $G$ is an object of $\mathbf{A}$. If there is a parent category $\mathbf{B}$ of $\mathbf{A}$ with an object $X$ and an injective morphism $i:X\to G$ satisfying that for any object $Y$ of $\mathbf{A}$ and each morphism $f:X\to Y$ there is a unique morphism $\tilde{f}:G\to Y$ such that the following diagram is commutative:

\begin{xy}

\xymatrix{

X\ar[r]^f \ar[d]_i&Y\\

G\ar[ur]_{\tilde{f}} & ,

}

\end{xy}

then $G$ is said to be a *free* object.

(discussed with S.Q.Huang)