"Churn" is a bit of jargon that companies use to describe the loss of customers in a given period, usually in a month. You'll hear it in press releases, analyst reports, and sales material from CRM vendors like "we brought our churn way down by using XYZ customer pleaser tool and our stock has been doing great ever since!"
I had heard about churn before, but never really dealt with it. For work I had to take a tool I was developing and replace an "expected customer lifetime in months" input for a "churn" input because "churn" is easier for our people to think about. That's fine, but what does "churn" mean when I'm calculating something over the life of a customer? My models needed a number of months.