The strict definition of CBD Isolate is it's CBD without any other phytocannabinoids including THC. It also has no terpenes or any of the other organic compounds that might be found in Industrial Hemp, Pharmaceutical, or Food Grade Hemp.
Some manufacturers consider CBD Isolate anything CBD and other phytocannabinoids excepting THC. Terpenes are not part of the formulation.
If a label says CBD Isolate but you have doubts, read the independent lab results. That should be available on the vendor’s website. The lab results will give you the percentages of phytocannabinoids present, or not present in the formulation. E.g., for any type of THC “ND” (for none detected) should be the amount listed. Again, the amount of phytocannabinoids besides THC may vary depending on how the manufacturer defines CBD Isolate.