Definition of second line
noun
1a battle line behind the front line to support it and make good its losses: its capture would allow them to penetrate the second line of enemy defences
2 [as modifier] ranking second in strength, effectiveness, ability, or value:second-line American computer manufacturers
3 [usually as modifier] a medical treatment or therapy used in support of another, or as a more drastic measure if the primary treatment is ineffective: second-line, expensive, often invasive tests
