Its not a reserved word in struts. It IS an attribute of an html form.
A lot of time in HTML you will see:
myForm.userName refers to the textfield on the form. eg myForm.userName.value = 'John Doe'
But what happens if you put a control called "action" in your form? Does myForm.action refer to the attribute "action" in the form tag, or the control embedded in the form? You see how the confusion arises?