Angel's Share
The portion of whiskey lost to evaporation through the barrel's wood every year it ages; commonly cited around 2% a year in Kentucky's climate, faster on a rickhouse's hot top floors.
Wood is porous, and a barrel breathes: as temperature swings, whiskey is drawn into the wood and pushed back out, and some of it escapes as vapor through the staves rather than returning to the liquid. That lost volume is the angel's share.
The rate isn't fixed; it depends on heat, humidity, and airflow, so it varies by rickhouse floor and by climate (a rickhouse in Kentucky loses whiskey differently than one in a hotter, drier climate like Texas). It's also why a barrel's proof can climb over time even though nothing is being added: in a hot, dry environment, water evaporates faster than alcohol, concentrating what's left.