Standby, reserve or extra.
Supporting, reinforcing; (music) of or related to accompaniment
That is intended as a backup.
reinforcements
A copy of a file or record, stored separately from the original, that can be used to recover the original if it is destroyed or damaged.
A reserve or substitute.
An accumulation of material caused by a (partial) obstruction or (complete) blockage of the flow or movement of the material, or an accumulation of material that causes an overflow due to the flow being greater than the maximum possible flow.
Second.
See other (determiner) below.
Alien.
Different.
To regard, label, or treat as an "other", as not part of the same group; to view as different and alien.
To treat as different or separate; segregate; ostracise.
Not the one or ones previously referred to.
An other, another (person, etc), more often rendered as another.
The other one; the second of two.