In (preposition)

1. Expressing the situation of something that is or appears to be enclosed or surrounded by something else.
2. Expressing a period of time during which an event happens or a situation remains the case.
3. Expressing the state of being involved with or preoccupied by something.
4. Expressing the means by which something is conveyed.
5. Expressing a ratio or comparison.

Origin:

Old english in, inn, of germanic origin; related to dutch and german in, also to inward and en-, of unknown ultimate origin.

Examples:

1. The keys are in the drawer.
2. She was born in the 90s.
3. He was lost in thought.
4. He communicated the message in a letter.
5. The ratio of boys to girls in the class is 3:2.