Which tag is used to define small text in HTML?

The tag used to define small text in HTML is the <small> tag. This tag semantically indicates that the text it encloses should be rendered in a smaller size than the surrounding text. This is often used for fine print or legal disclaimers, where the differentiation in size helps communicate that the enclosed text is less important or secondary compared to the main content. The <small> tag is widely supported across browsers and contributes to better accessibility since screen readers can interpret its meaning appropriately.

The other options do not represent valid HTML tags. <textsmall>, <mini>, and <tiny> are not recognized by HTML and therefore would not render any desired effect in a webpage. Therefore, the <small> tag stands out as the correct choice for defining smaller text in HTML.