Meaning of “solemn” - Learner’s Dictionary

solemn

adjective uk us /ˈsɒləm/
SERIOUS

serious or sad:

solemn music
Everyone looked very solemn.
PROMISE/WARNING ETC

A solemn promise, warning, etc is serious and sincere.

solemnly adverb
solemnity uk us /səˈlemnəti/ noun [ U ]

(Definition of “solemn” from the Cambridge Learner’s Dictionary © Cambridge University Press)