Second has a number of different meanings:

Angle:

Unit of Angle: The arcsecond or second of arc (symbol: ″)

Unit of Angle: The second of right ascension (symbol: s)

Democracy:

In relation to democratic procedure:

• To second is to support a proposal.

Duelling and fighting:

The second is the agent of the person who submits a challenge to someone who proposes a duel or the agent of the person responds to said challenge.

Music:

In relation to music the intervals:

• Augmented second
• Major second
• Medium second
• minor second
• all being linear steps and vertical dissonances (with the possible exception of the medium second/minor third)

Numbers

• Also medium second.

The second item or entry is the number 2 entry, following first.

Rock Climbing:

In relation to rock climbing:

• The second is the climber who follows the lead climber.

Time:

Unit of time: the second