A conjunction that joins two grammatical elements of the same status or construction
Or is a coordinating conjunction in the sentence I can't tell whether it's day or night, day and night both being singular nouns.
a word such as 'and' or 'but' which joins two words, groups, or clauses which are equal in importance or rank subordinating conjunction
a conjunction (like `and' or `or') that connects two identically constructed grammatical constituents the coordination by conjunction of linguistic units of the same status