solemn

Short definition from Wiktionary

Adjective

1. 1
   Of or pertaining to religious ceremonies and rites; (generally) religious in nature; sacred.
2. 2
   Characterized by or performed with appropriate or great ceremony or formality.
3. 3
   Deeply serious and sombre; grave.
4. 4
   Inspiring serious feelings or thoughts; sombrely impressive.
5. 5
   Cheerless, gloomy, sombre.