Credibility comprises the objective and subjective components of the believability of a source or message. Credibility dates back to Aristotle theory of Rhetoric. Aristotle defines rhetoric as the ability to see what is possibly persuasive in every situation. He divided the means of persuasion into three categories, namely Ethos (the source's credibility), Pathos (the emotional or motivational appeals), and Logos (the logic used to support a claim), which he believed have the capacity to influence the receiver of a message. According to Aristotle, the term "Ethos" deals with the character of the speaker. The intent of the speaker is to appear credible. In fact, the speaker's ethos is a rhetorical strategy employed by an orator whose purpose is to "inspire trust in his audience." Credibility has two key components: trustworthiness and expertise, which both have objective and subjective components. Trustworthiness is based more on subjective factors, but can include objective measurements such as established reliability. Expertise can be similarly subjectively perceived, but also includes relatively objective characteristics of the source or message (e.g., credentials, certification or information quality). Secondary components of credibility include source dynamism (charisma) and physical attractiveness.

Credibility online has become an important topic since the mid-1990s. This is because the web has increasingly become an information resource. The Credibility and Digital Media Project @ UCSB highlights recent and ongoing work in this area, including recent consideration of digital media, youth, and credibility. In addition, the Persuasive Technology Lab at Stanford University has studied web credibility and proposed the principal components of online credibility and a general theory called Prominence-Interpretation Theory.