What language is used to describe Jenkins pipeline?

Jenkins Pipeline is described using Groovy DSL (Domain Specific Language). It is a scripting language based on Groovy that allows you to describe build, test, and deployment stages as code. An example of a simple Jenkinsfile in Groovy DSL:

pipeline {
    agent any
    stages {
        stage('Build') {
            steps {
                echo 'Building...'
            }
        }
        stage('Test') {
            steps {
                echo 'Testing...'
            }
        }
    }
}