As far as I'm concerned,
- an equation is any mathematical statement of the form LHS=RHS,
- an inequality is any mathematical statement of the form LHS?RHS (where ? is a placeholder for some inequality sign),
- an expression is any mathematical quantity (it contains no equal sign or inequality sign).
LaTeX's built-in cross-referencing to maths stuff can of course be customised with dedicated packages (such as
cleveref), but it seems to me that very few TeX users actually choose to reflect that distinction in their documents; most seem quite happy to use the name Equation throughout. That bothers me a bit...
Do you think that one should use different cross-referencing names for equations, inequalities, and expressions?
Besides, what about the singular form incorrectly used instead of the plural one? Here is a baby example:
$a \leq x \leq b$
Here, we actually have, not one, but two inequalities. Isn't it wrong/confusing to refer to it as a single inequality (not to mention a single equation)?
Note: I'm conscious these questions may seem primarily opinion-based, but I feel that a limited number of those is acceptable on the site. After all, I'm sure esteemed members of the community would agree that promoting good composition style is at least part of the site's raison d'être. Feel free to disagree and migrate the question to meta or a community poll.