Out of stock at the store level means a given product is not available at a given time. 
Out-of-Stocks (or Stock-Outs) quantify the number of retail outlets where an item is expected to be available for customers but is not. It is typically expressed as a percentage of stores that list the relevant item. 
Out-of-stocks (%) = Outlets Where Brand or Product is Listed But Unavailable (#) / Total Outlets Where Brand or Product is Listed (#)
- MASB Common Language in Marketing Project, 2020.
- Farris, Paul W.; Neil T. Bendle; Phillip E. Pfeifer; and David J. Reibstein (2010). Marketing Metrics: The Definitive Guide to Measuring Marketing Performance (Second Edition). Upper Saddle River, New Jersey: Pearson Education, Inc.