An internal link is a type of hyperlink on a webpage to another page or resource, such as an image or document, on the same website or domain. Hyperlinks are considered either "external" or "internal" depending on their target or destination. Generally, a link to a page outside the same domain or website 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 be classified as "internal" for some purposes. Ultimately, an internal link points to a web page or resource in the same root directory.
Similarly, seemingly "internal" links are in fact "external" for many purposes, for example in the case of linking among sub domains of a main domain, which are not operated by the same person. For example, a blogging platform, such as Word Press, Blogger or Tumbler host thousands of different blogs on sub domains, 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.