Glossophobia or speech anxiety is the fear of public speaking. The causes of glossophobia are uncertain but explanations include communibiology and the illusion of transparency. Considerable research has been conducted into the causes of glossophobia, with a number of potential causes being suggested. One proposed explanation is that these anxieties are a specific symptom of social anxiety produced by fearfulness related to the fight-or-flight response, which is produced by a perceived threat; this triggers an elevated defense reaction in the Sympathetic nervous system to be alert, to run, hide or freeze.

Source: Wikipedia