The term 'firstborn' is used to refer to the first child born in a family. The crossword solutions for this term are SENIOR (6 letters), ELDER (5 letters), OLDEST (6 letters), ELDEST (6 letters), and CAIN (4 letters). SENIOR and ELDER both refer to someone who is older than others, while OLDEST and ELDEST both refer to someone who is the OLDEST in a group. Finally, CAIN is a biblical reference to the first son of Adam and Eve, making him the firstborn of the human race.