A USPS cachet is one produced by the United States Postal Service.

Examples are:

Scott 1396 - USPS Inauguration, FDOI postmarked by every US Post Office on 7/1/1971 when the USPS ceased being a US government entity and became an independent entity.  (cachet has the USPS logo).  "First Day" postmark in Washington D.C.

Scott 1489-1498 - Postal People, FDC postmarked 4/20/1973.  (cachet has the USPS Logo)

Scott 2419 - Priority Mail, 20th anniversary of moon landing, FDC postmarked 7/20/89.  (cachet has a picture of an astronaut stepping from the lander)