the shell around the kernel of a nut
"She cracked open the walnut to separate the meat from the hard nutshell before eating it."
The shell that surrounds the kernel of a nut.
"She cracked open the walnut to remove the bitter nutshell before eating the soft kernel inside."
To summarize (from the term in a nutshell).
"In a nutshell, our project succeeded because we finally streamlined the approval process."