What does it mean to "default" on my federal student loans?
Default is the failure to repay a loan according to the terms agreed to in the promissory note. For most federal student loans, you will default if you have not made a payment in more than 270 days.
If you haven’t made a payment on your federal student loan for 270 days (nine months), and have not made arrangements with your lender or servicer that do not obligate you to make those payments, like deferment or forbearance, you are probably in default. During the months in which you have failed to make payments on your federal student loans, your loan holder must exercise "due diligence" in attempting to collect the loan - your loan holder must make repeated efforts to locate and contact you about repayment.
If you have not received a letter from your servicer and you believe you may be in default, you should contact your servicer immediately to discuss repayment options and determine if it is possible for you to avoid default.