"Alternate days" means to switch between two options or activities every other day. For example, if you exercise on alternate days, it means you exercise one day and take a break the next, and continue the pattern. Full definition
Another study on alternate day fasting found a decrease in resting metabolic rate of 59 calories per day. (completehumanperformance.com)
Since moving home I've gotten my mom on the green smoothie train - banana, spinach, blueberries, almond milk - which we basically alternate every day with oatmeal. (hummusapien.com)
Perform this workout on alternating days for four to six week. (fitnessandpower.com)