Innermost, outermost, foremost, and hindmost are related words that describe the most extreme place in a direction or position.
Innermost means farthest inside. You might say the innermost ring or the innermost room.
Outermost means farthest outside. For example, the outermost layer is the layer on the very outside.
Foremost has two common uses. It can mean most important, as in Safety is our foremost concern. It can also mean at the front, though this use is a little less common in everyday speech.
Hindmost means farthest back. For example, the hindmost runner is the runner at the back of the group.
- inside: innermost
- outside: outermost
- front or highest importance: foremost
- back: hindmost
A useful pattern is that these words often mean most in some position. They are especially common in careful, descriptive writing.