I have a question in US History?

My teacher gave us a crossword for review and I have to complete it for homework. This is what it says as the hint: Plan of government which provided for three branches and a strong federal government. Please help and don't answer rudely. If you don't know the answer to the question, then don't answer. Thank you for your time. Whoever gets the right answer first is rewarded 10 points. I know I already asked this question but for those who already answered there is more detail now. The word is 8 letters and the fifth letter is I.