JSP <c:if> Tag: Conditional Logic in JavaServer Pages

The '<c:if>' tag is used in JSP (JavaServer Pages) to add conditional logic to a page. It allows you to test a condition and execute a block of code if the condition is true.

Here is an example:

<c:if test='${param.username != null}'>
   Welcome, ${param.username}!
</c:if>

In this example, the code between the '<c:if>' and '</c:if>' tags will only be executed if the 'username' parameter is not 'null'. The '${param.username}' expression is used to retrieve the value of the 'username' parameter from the HTTP request.