MUV stands for Monthly Unique Visitor. A monthly unique visitor is an individual user who accesses your site within 30-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.