Definition of semiformal in US English:
 semiformal
adjectiveˌsemēˈfôrməlˌsemˌī-
1Combining formal and informal elements.
 in the semiformal atmosphere irritations can be aired
 Example sentencesExamples
-  Sometimes the biggest challenge to getting ready for a semi-formal date is defining ‘semiformal.’
 -  My wedding was semiformal, so everyone just dressed neat and tidy.
 -  Today, it's another iteration of ‘How to dress yourself,’ this time for semiformal events such as… well, this work function, or a nice party.
 
- 1.1 Used to describe clothing that is neither formal nor casual and that is typically worn for a dance, wedding, or other event.
 the casino has a semiformal dress code (men must wear a tie and jacket)
 Example sentencesExamples
-  Most dress or semiformal trousers should have a slight to medium break when the bottom meets the shoe, no more than one inch.
 -  If the wedding in question takes place before 6:30 or 7:00 at night, there's a good chance that the bride and groom are skipping black tie and opting instead for semiformal dress.
 -  Patients prefer doctors to wear semiformal dress and are most comfortable with conservative items.
 
 
nounˌsemēˈfôrməlˌsemˌī-
An event at which semiformal attire is expected.
 it had organized its own spring semiformal at a nearby hotel
 Example sentencesExamples
-  I met him at the semiformal that Scott was supposed to attend with me.
 -  My school has what we call the semiformal (semi, for short), which is held in December.
 -  As I mentioned, I invited him to be my date for the semiformal of my junior year.
 -  Later that evening the Kimmel Center hosts a red-carpet-styled gala for the glitterati and a dress-to-the-nines semiformal.