Substitute/ˈsʌbstɪtuːt/ Definition 1 of 4Pro Tip 1/3Temporary Replacement Refers to someone who takes a place for a short time, not permanently. Sara was a substitute teacher for just one week. Pro Tip 2/3Not Identical The substitute might be similar but not exactly the same as what it replaces. Almond milk can be a good substitute for cow's milk in recipes. Pro Tip 3/3Plan B A substitute often acts as a backup option when the original is unavailable. An in-class assignment was the substitute for the canceled field trip.