Conjunction
- Because event X may occur
- I'll take an umbrella, in case it rains. (i.e. because it may rain later)
Preposition
- Because of the possibility of
- Take an umbrella in case of rain. (i.e. even if it's not raining, take an umbrella because it may rain later)
- In the event of
- In case of emergency, break glass. (i.e. if and only if there is an emergency)
Origin
Middle English
