Embark
Embark (colony) in the Stellaris encyclopedia: To Embark is when a Settled Empire leaves their Habitable World and boards an Arkship to become a Nomadic Empire.
Embark in the game's words
The encyclopedia entry as the game shows it in its tooltips; underlined words open their own entry or page here.
To 
Embark is when a 
Settled Empire leaves their Habitable World and boards an 
Arkship to become a 
Nomadic Empire.
This requires that either the Adaptability or Versatility tradition tree has been completed.
To Embark, open the Decisions panel on a 
Colony and select Embark.
If an empire has multiple Colonies they can choose to leave them behind as a Subject, or instead release the new Nomadic Empire as a Subject.
All owned Starbases are converted into 
Waystations, and the embarked planet becomes a Stripped World.
More in Colony
Source: common/game_concepts (concept_arkship_embark, also concept_arkship_embarkation, concept_arkship_embarking) and localisation/english, Stellaris Cygnus v4.5.2 (9776).