"Excellence, then, is not an act but a habit."
The original quote is much longer, but sometimes I think it is helpful to break things down a bit. Can the word excellence really not denote an act, but actually be understood as a habit? What does this mean? The idea here is that by acting appropriately each and every day, we will achieve excellence. So, if excellence is ‘won’ by acting rightly again and again, then it follows that excellence is actually a habit, not one act, but a habit.