core; nucleus

You two are the nucleus of the team.

The heart of Esperanto ideology is the equality of all languages and cultures.

On a clear October day, Naini Lake shimmers in the afternoon sun as boatmen row tourists across the hill resort’s prime attraction nestled amid lush, pine-clothed Himalayan peaks.

In my soul there are many places that are cynical and arrogant, but at its core, it fears the darkness and is as weak as a sheep.