serious in WordNet English dictionary

1. causing fear or anxiety by threatening great harm
   "a dangerous operation"; "a grave situation"; "a grave illness"; "grievous bodily harm"; "a serious wound"; "a serious turn of events"; "a severe case of pneumonia"; "a life-threatening disease"
   dangerous  grave  grievous  serious  severe  life-threatening 
   
2. appealing to the mind
   "good music"; "a serious book"
   good  serious 
   
3. concerned with work or important matters rather than play or trivialities
   "a serious student of history"; "a serious attempt to learn to ski"; "gave me a serious look"; "a serious young man"; "are you serious or joking?"; "Don't be so serious!"
   serious 
   
4. completely lacking in playfulness
   unplayful  serious  sober 
   
5. requiring effort or concentration; complex and not easy to answer or solve
   "raised serious objections to the proposal"; "the plan has a serious flaw"
   serious 
   
6. of great consequence
   "marriage is a serious matter"
   serious