WordNet
adj. moderately formal; requiring a dinner jacket; "he wore semiformal attire"; "a black-tie dinner" [syn: semiformal, black-tie]
Wikipedia
In Western clothing semi-formal is a grouping of dress codes indicating the sort of clothes worn to events with a level of protocol between informal (e.g., lounge suit) and formal. Both morning and evening semi-formal attire share design features in common with the informal lounge suit.
Usage examples of "semi-formal".
He was dressed in a semi-formal outfit of loose pantaloons and long shirt, and enjoying the lushly sensual sensation of the glidingly smooth materials on his body.
A section of curved wall became a mirror and the Consul stared at the image there: a more than middle-aged man in semi-formal evening wear, sunburned skin but oddly pale under the sad eyes.