Jenkinsfile variable used in two separate stages

In a declarative pipeline, @mkobit's answer won't work. You can, however, switchinto script mode explicitly and use its scoping, e.g. like so:

...
steps {
   script {
       def foo = sh script: "computeFoo", returnStdout: true

       node('name') {
           script {
               someStep()
           }
       }
    }
}
...

Just define the variable before your node block:

def rev = ''
node('build') {
  stage('Checkout') {
    checkout scm
  }
  stage('Build') {
    bat(script: 'build')
    rev = readFile('result')
  }
}