But SDR on it's own could stand for Strategic Defense Review....
The SRD (as above) is the core set of (RuneQuest) rules published by Mongoose which are covered by the OGL (Open Gaming License). It basically defines what can be included in OGL-compliant games or supplements.
There are other SRDs, the most well-known being the various incarnations of the d20 SRD (d20, d20 Modern, and d20 SF (or Modern + SF, really)).
For example "Bigby's Clenched Fist" is not in the d20 SRD and should not be be used in d20 products, only in WotC products. However, "Clenched Fist" _is_ a level 8 Strength spell and is in the d20 SRD.