MUV stands for Monthly Unique Visitor. A monthly unique visitor is an individual user who accesses your site within 21-day window. The unique visitors is determined by a cookie that lasts for a period of 1 year or until users clear their browser cookies for websites.
For mobile apps, the equivalent is a monthly active user - an app user who opened your app at least once within the 30-day window. A unique app user is determined as the unique instance of the app installation until the user's app data is cleared or the app is uninstalled.
On the Sprout plan, there is a hard limit of 10,000 MUV. On Blossom, Garden and Estate, your account will not be charged anything for upto 200K MUV. Once your MUV limit crosses 200K, your account will be charged $50 per month for every additional 100K above 200K. Let us look at some example cases:
CASE 1: Your current MUV is 160K -> you are not charged anything
CASE 2: Your current MUV is 350K -> you will be additionally charged $100 ($50 for 200-300K and another $50 for 300-40K)