Proper citation of equations in scientific papers - terminology Most scientific papers refer to almost any formula that contains the "=" sign by the word "equation". Consider this case:

Let value = function(input) (1) where input is a known input value and value is the result of the function computation. In this situation, there are no effective unknowns (we are guaranteed that no unknowns are hidden in the function expression either).

Most papers (if not all) often refer to expression (1) in sentences like "equation (1) is equivalent to", "referring to equation (1), we see that..". 

Some pedantic referees, however, suggest the use of a more proper word (e.g. formula (1) instead of equation (1)).

Is there a grammar reference that solves this seemingly insignificant issue?

Further clarification

An  example of an expression not considered an equation by a referee:
(a+b)^2 = a^2 + 2ab + b^2 (1).
From Higham, Handbook of writing for the mathematical sciences (emphasis mine):

Referencing Equations
When you reference an earlier equation it helps
the reader if you add a word or phrase describing the nature of that
equation. The aim is to save the reader the trouble of turning back to
look at the earlier equation. For example, &quot;From the definition (6.2)
of dual norm&quot; is more helpful than &quot;From (6.2)&quot;; and &quot;Combining the
recurrence (3.14) with inequality (2.9)&quot; is more helpful than
&quot;Combining (3.14) and (2.9)&quot;. Mermin [200] calls this advice the &quot;Good
Samaritan Rule&quot;. As in these examples, the word added should be something more informative than just &quot;equation&quot; (or the ugly abbreviation &quot;Eq.&quot;), and inequalities, implications and lone
expressions should not be referred to as equations.

Pro-tip: arguing with a referee on such trivialities is rarely worth your time. Just do what they suggest.