Entity Validation API: Retrieve parent field entity while validating
By using FieldItemList::getEntity like in this core validator:

Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldValueValidator:

class UniqueFieldValueValidator extends ConstraintValidator {

  /**
   * {@inheritdoc}
   */
  public function validate($items, Constraint $constraint) {
    if (!$item = $items-&gt;first()) {
      return;
    }
    $field_name = $items-&gt;getFieldDefinition()-&gt;getName();
    /** @var \Drupal\Core\Entity\EntityInterface $entity */
    $entity = $items-&gt;getEntity();


The first argument of a validate method is of mixed type. For the Drupal Entity Validation API this can be a field value, a field item list or an entity, depending on where you add the constraint.