How can I use `def` in jenkins pipeline

You can use def with a declarative pipeline, just not inside it eg

def agentLabel
if (BRANCH_NAME =~ /^(staging|master)$/)  {
    agentLabel = "prod"
} else {
    agentLabel = "master"
}

pipeline {
  agent { node { label agentLabel } } 
..

As @Rob said, There are 2 types of pipelines: scripted and declarative. It is like imperative vs declarative. def is only allowed in scripted pipeline or wrapped in script {}.

Scripted pipeline (Imperative)

Start with node, and def or if is allowed, like below. It is traditional way.

node {
    stage('Example') {
        if (env.BRANCH_NAME == 'master') {
            echo 'I only execute on the master branch'
        } else {
            echo 'I execute elsewhere'
        }
    }
}

Declarative pipeline (Preferred)

Start with pipeline, and def or if is NOT allowed, unless it is wrapped in script {...}. Declarative pipeline make a lot things easy to write and read.

time trigger

pipeline {
    agent any
    triggers {
        cron('H 4/* 0 0 1-5')
    }
    stages {
        stage('Example') {
            steps {
                echo 'Hello World'
            }
        }
    }
}

when

pipeline {
    agent any
    stages {
        stage('Example Build') {
            steps {
                echo 'Hello World'
            }
        }
        stage('Example Deploy') {
            when {
                branch 'production'
            }
            steps {
                echo 'Deploying'
            }
        }
    }
}

Parallel

pipeline {
    agent any
    stages {
        stage('Non-Parallel Stage') {
            steps {
                echo 'This stage will be executed first.'
            }
        }
        stage('Parallel Stage') {
            when {
                branch 'master'
            }
            failFast true
            parallel {
                stage('Branch A') {
                    agent {
                        label "for-branch-a"
                    }
                    steps {
                        echo "On Branch A"
                    }
                }
                stage('Branch B') {
                    agent {
                        label "for-branch-b"
                    }
                    steps {
                        echo "On Branch B"
                    }
                }
            }
        }
    }
}

embedded with scripted code

pipeline {
    agent any
    stages {
        stage('Example') {
            steps {
                echo 'Hello World'

                script {
                    def browsers = ['chrome', 'firefox']
                    for (int i = 0; i < browsers.size(); ++i) {
                        echo "Testing the ${browsers[i]} browser"
                    }
                }
            }
        }
    }
}

To read more declarative pipeline grammar, please refer the official doc here