Conditional step/stage in Jenkins pipeline

I'm adding this answer to explicitly mention using a condition in a step in contrast to a stage, both within declarative pipelines.

Condition in a stage of a declarative pipeline

As already shown by @Chandan Nayak and others, this can be done based on when as in

stage('myConditionalStage') {
    when {
        branch 'myBranch'
    }
    steps {
        echo 'triggered by myBranch'
    }
}

So the stage myConditionalStage will only be run if triggered by a push to myBranch.

Condition in a step of a declarative pipeline's stage

If you however need a condition within the steps section of a stage, you can use Groovy syntax (if/else in this case) that is used in Scripted pipelines. In case of a Declarative pipeline you have to put it into a script block as follows:

stage('myStage') {
    steps {
        echo 'within myStage'
        script {
            if (env.BRANCH_NAME == "myBranch") {
                echo 'triggered by myBranch'
            } else {
                echo 'triggered by something else'
            }
        }
    }
}

For a Scripted pipeline, you can use it without the script block as shown by @Krzysztof Krasoń

According to other answers I am adding the parallel stages scenario:

pipeline {
    agent any
    stages {
        stage('some parallel stage') {
            parallel {
                stage('parallel stage 1') {
                    when {
                      expression { ENV == "something" }
                    }
                    steps {
                        echo 'something'
                    }
                }
                stage('parallel stage 2') {
                    steps {
                        echo 'something'
                    }
                }
            }
        }
    }
}

Doing the same in declarative pipeline syntax, below are few examples:

stage('master-branch-stuff') {
    when {
        branch 'master'
    }
    steps {
        echo 'run this stage - ony if the branch = master branch'
    }
}

---

stage('feature-branch-stuff') {
    when {
        branch 'feature/*'
    }
    steps {
        echo 'run this stage - only if the branch name started with feature/'
    }
}

---

stage('expression-branch') {
    when {
        expression {
            return env.BRANCH_NAME != 'master';
        }
    }
    steps {
        echo 'run this stage - when branch is not equal to master'
    }
}

---

stage('env-specific-stuff') {
    when { 
        environment name: 'NAME', value: 'this' 
    }
    steps {
        echo 'run this stage - only if the env name and value matches'
    }
}

More effective ways coming up - https://issues.jenkins-ci.org/browse/JENKINS-41187
Also look at - https://jenkins.io/doc/book/pipeline/syntax/#when

---

The directive beforeAgent true can be set to avoid spinning up an agent to run the conditional, if the conditional doesn't require git state to decide whether to run:

when { beforeAgent true; expression { return isStageConfigured(config) } }

Release post and docs

---

UPDATE
New WHEN Clause
REF: https://jenkins.io/blog/2018/04/09/whats-in-declarative

equals - Compares two values - strings, variables, numbers, booleans - and returns true if they’re equal. I’m honestly not sure how we missed adding this earlier! You can do "not equals" comparisons using the not { equals ... } combination too.

changeRequest - In its simplest form, this will return true if this Pipeline is building a change request, such as a GitHub pull request. You can also do more detailed checks against the change request, allowing you to ask "is this a change request against the master branch?" and much more.

buildingTag - A simple condition that just checks if the Pipeline is running against a tag in SCM, rather than a branch or a specific commit reference.

tag - A more detailed equivalent of buildingTag, allowing you to check against the tag name itself.

Just use if and env.BRANCH_NAME, example:

    if (env.BRANCH_NAME == "deployment") {                                          
        ... do some build ...
    } else {                                   
        ... do something else ...
    }