Is the cardinality of an ordinal, $\alpha$ less than $\alpha$?

Question

Let $k^+=\{\alpha : \alpha \hspace{2mm} \text{is an ordinal,and} \hspace{2mm} |\alpha|<\kappa\}$ where $k^+$ is an infinite cardinal.

I am trying to understand the proof the the following statement:

If $\kappa$ is a cardinal, then $\kappa^+$ is the smallest cardinal which is bigger than $\kappa$.

The proof begins with the following statement,

If $\alpha < \kappa^+$, then $|\alpha| \le \alpha < \kappa^+$.

I am not sure how to interpret $| \alpha| \le \alpha$. The cardinality of an ordinal is less than the ordinal? So, the cardinal $\kappa$ for which there exists a bijection $f: \kappa \to \alpha$ is such that $\kappa \subseteq \alpha$?

Answer 1

First of all, recall that $|\alpha|$ is an ordinal itself which satisfy that for all $\beta<|\alpha|$, there is no bijection from $\beta$ onto $|\alpha|$.

And by definition $|\alpha|$ is also the least ordinal such that $|\alpha|$ and $\alpha$ have a bijection. In particular it cannot be the case that $\alpha<|\alpha|$. Therefore $|\alpha|\leq\alpha$.

(That been said, in the definition of $\kappa^+$ it should really be $|\alpha|\leq\kappa$, as Daniel Fischer remarks in the comments.)