sem·i·for·mal 
(sĕm′ē-fôrməl, sĕm′ī-)
(sĕm′ē-fôrməl, sĕm′ī-)adj.
1. Moderately formal: a semiformal dance.
2. Suitable or appropriate for a moderately formal occasion: semiformal attire.
n.
Something, such as a dance, that is semiformal in nature.