Many GUI building platforms, like Swing and Visual Basic have a form widget that acts as a combination select list (dropdown list) and text input field. Users of this control can either select a predefined entry from the list or they can type in their own value. The 'Combo' in ComboBox (or JComboBox) comes from the word 'combination' and is used the describe this control which provides two ways for an end user to enter or select a value.
HTML does not have any such control. It has a select list which can be used to select from pre-defined values and it has an input field of type 'text' in which the user can type their own values.
Calling a select list a ComboBox in the HTML or JSP forums can potentially create a RedHerring which makes it less clear to people trying to help you whether or not you understand the nature of the control. By making sure that you effectively convey both the nature of your problem and your level of understanding to the readers of your question, you are helping to insure that your question is in the best position possible to be quickly and accurately answered.