Diastatic power is a malt's enzyme content, measured in degrees Lintner (°L) — a malt with meaningful diastatic power carries enough active enzyme to convert its own starch (and sometimes a portion of an adjunct's starch too) during the mash, while a malt with zero or near-zero diastatic power relies entirely on enzyme borrowed from whatever base malt sits alongside it in the grist.
Why this matters for specialty and adjunct grain specifically
Base malts (Pilsner, Pale 2-Row, Maris Otter, Munich malt) generally carry substantial diastatic power on their own. Specialty and roasted malts — crystal, chocolate, black patent, roasted barley — are kilned or roasted hot enough that their own enzymes are destroyed in the process, meaning they contribute flavor and colour but zero conversion power of their own; they depend entirely on the base malt's enzyme reserve to convert whatever starch they still carry.
The practical limit this puts on a grain bill
Adjuncts like flaked corn, flaked oats, or raw wheat also carry no enzyme power of their own and lean on the base malt's diastatic power the same way specialty malts do — which is why a grain bill heavy on adjunct or specialty grain, without enough high-diastatic-power base malt to cover the whole bill's conversion needs, risks an incomplete mash and a lower-than-expected gravity even at correct temperature and timing.