An example of programmatically creating a term reference entity field:

$fields[] = BaseFieldDefinition::create('entity_reference')
 ->setLabel(t(<label>))
 ->setDescription(t(<description>))
 ->setRevisionable(TRUE)
 ->setSetting('target_type', 'taxonomy_term')
 ->setTranslatable(TRUE)
 ->setTargetEntityTypeId(<vocabulary machine name>);

 

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