An isolate refers to a cannabinoid that has been purified to a high degree, with the surrounding plant matrix of other cannabinoids, terpenes and phytochemicals removed. Cannabidiol isolate, for example, consists of cannabidiol in near-pure crystalline or powdered form.
Isolates are notable in pharmacology because they allow a single chemical entity to be measured and standardised precisely. This makes dosing and characterisation more straightforward than with multi-compound extracts.
The trade-off, as discussed in the literature, is that an isolate by definition lacks the additional plant compounds present in full-spectrum or broad-spectrum preparations. Whether that matters is connected to the unresolved question of the entourage effect.