Second Language is

The correct option is: Any Language Other Than The Learner’s Native Language Or Mother TonguE)

Understanding First vs. Second Language:
In the field of linguistics and language acquisition, languages are categorized based on the order in which they are acquired:
1. First Language (L1): Also referred to as the mother tongue or native language, this is the language (or languages) a person acquires naturally from birth or early childhood through immersion.
2. Second Language (L2): This refers to any language that a learner starts to acquire after their native language is established. Despite the word "second," it represents any additional language learned later in life (whether it is the second, third, or fourth language).

Why the Other Options Are Incorrect:
• The Language Second In Importance...: A second language is defined chronologically (learned after the first language), not by its utility or importance to the speaker.
• The Language Used By A Second-Generation Learner...: This option confuses linguistic definitions with sociological terms regarding immigrant generations.
• The Language Which Is The Learner’s Native Language But Not The Mother Tongue...: "Native language" and "mother tongue" are synonymous terms representing the first language, meaning a second language cannot be either of them.