You downloaded a definition file for SEP 14.x but are running SEP 12.x. Or, you mixed 32-bit/64-bit. Fix: Check your SEP version: Open SEP interface > Help > About. Ensure you download the matching file (e.g., v5i64 for 14.x 64-bit). Legacy SEP 11/12 uses x64.exe (not v5i64).
Would I recommend it? – Yes for enterprise IT; no for casual home users (try a consumer AV instead). symantec endpoint protection virus definitions download
For administrators managing multiple clients through Symantec Endpoint Protection Manager (SEPM), downloading .jdb files is the standard method for updating the entire network's repository. You downloaded a definition file for SEP 14