Accessing variable in code behind from markup html

I'd like to access a variable I declare in the code behind file using <% %> tags. In the past I tried just doing
dim strUsername as string  <--- in the codebehind
<% = strUsername %> in the html markup
This worked well but when only one user accessed the page. When several users hit the same page at the same time they would get each others usernames displayed. How do I declare the variable so that only the user calling the page will have his variable displayed.
Also, what is the technical term for <%  %> code?

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