A COA is a Certificate of Analysis. Sometimes also called a Cert or Certificate. It contains the results from any testing done on the batch of materials being shipped.
This certificate is different than a specification sheet. The Specification (or spec) tells you what SHOULD be in the material, often reported as a range. The COA tells you what's actually in the batch you are receiving. For example, if purchasing a bag of sugar:
Specification: 800-850 kg/m3
COA: LOT 12345, Processed 1/28/19, 834 kg/m3