Is there a word meaning "the other side to compare against"?

Solution 1:

The standard against which something is compared is a comparator.

comparator noun
...
something used as a standard for comparison:
even taking the most favourable comparator the company is about 20 per cent higher

[ODO]

Solution 2:

Comparand is the term used in technical contexts for the objects being compared, similarly to operand. – Kris May 3 '13 at 12:03

It means

(linguistics, computing) A thing to which something is compared.

[Wiktionary]

Comparator, on the other hand, is the thing (device or algorithm) that performs the comparison.

Solution 3:

In a scientific context, this could be a Control.

A scientific control is an experiment or observation designed to minimize the effects of variables other than the independent variable.1 This increases the reliability of the results, often through a comparison between control measurements and the other measurements. Scientific controls are a part of the scientific method. Wikipedia

In literature, you could use the word Foil

In fiction, a foil is a character who contrasts with another character (usually the protagonist) in order to highlight particular qualities of the other character. Wikipedia

Solution 4:

Depending on the context, a benchmark may work too.

ODO:

benchmark NOUN

1 A standard or point of reference against which things may be compared.

‘the pay settlement will set a benchmark for other employers and workers’

‘There is no question that this new plant has set new standards in every aspect and will unquestionably become a benchmark for future factories in the future.’