There are two possible definitions for Secunda that I know of:
1. The ambiance track composed by
Jeremy Soule.
2. The Smaller moon of the two moons of
Nirn which orbits
Masser, the larger moon.
Person 1: Hey, have you heard Secunda?
Person 2: I Haven't.
Person 1: Have you played
The Elder Scrolls V: Skyrim?
Person 2: Yeah.
Person 1: It's one of the
ambience tracks in The Elder Scrolls V: Skyrim.