An internal link is a type of hyperlink on a webpage. Links are considered either "external" or "internal" depending on their target. Generally, a link to a page outside the same domain is considered external, whereas one that points at another section of the same webpage or to another page of the same website or domain is considered internal.
However, these definitions become clouded when the same organization operates multiple domains functioning as a single web experience, e.g. when a secure commerce website is used for purchasing things displayed on a non-secure website. In these cases, links that are "external" by the above definition can conceivably classified as "internal" for some purposes.
Similarly, seemingly "internal" links are in fact "external" for many purposes, for example in the case of linking among subdomains of a main domain, which are not operated by the same person(s). For example, a blogging platform, such as Wordpress, Blogger or Tumblr host thousands of different blogs on subdomains, which are entirely unrelated and the authors of which are generally unknown to each other. In these contexts one might view a link as "internal" only if it linked within the same blog, not to other blogs within the same domain.