Relational operators such as <, >=, == are often not bidirectional. What are the correct names for the operands to these operators? An example from an non-relational operator, division, would be the operand names divisor and dividend.
Comparand for either operand.
An operand of an equality/inequality is referred to as a comparand in some mathematical, linguistic & computing contexts. Google 'comparand' with 'math', 'linguistics', 'ibm', 'intel', 'microsoft' or 'site:stackoverflow.com'.
(It's not in the OED or Merriam-Webster. It's in the Wiktionary.)
(For 'linguistics' Google even finds a comparee & comparand.)