Literally: purple – electricity – one – flash
Alternately: The light that glints from a sword being brandished. By extension, an extremely short moment of time; a single instant. By extension, a dramatic change that happens in very little time.
Notes: This is a compound of compounds; 紫電 is the purple light that glints off of a sharp blade. (Why purple? I’m not sure.) 一閃 is a single, momentary flash.
Long-time readers may recall another yojijukugo on a similar theme… twice.