Its whatever your DM decides it is
An object is not defined in D&D 5e except by context and by what it is not: a creature is not an object, a situation is not an object etc. Therefore, we have to turn to common English usage. Feel free to use whatever dictionary you choose, this is from the Oxford English dictionary:
A material thing that can be seen and touched.
1.1 Philosophy A thing external to the thinking mind or subject.
A person or thing to which a specified action or feeling is directed.
2.1 A goal or purpose.
Grammar A noun or noun phrase governed by an active transitive verb or by a preposition.
Computing A data construct that provides a description of anything known to a computer (such as a processor or a piece of code) and defines its method of operation.
D&D 5e is probably using definition 1 (although the exclusion of creatures may make it more 1.1). Now, we can follow on to the definition of a thing but it doesn't really illuminate much.
Considered philisophically, "an object may be considered as not having real or full existence or value independent of the subject who observes it." In particular:
The pragmatist Charles S. Peirce defines the broad notion of an object as anything that we can think or talk about. In a general sense it is any entity: the pyramids, Alpha Centauri, the number seven, a disbelief in predestination or the fear of cats. In a strict sense it refers to any definite being.
The general sense doesn't seem to be what D&D is thinking about but the strict sense may be.
Before considering the vagaries of liquids or gasses lets just look at the difficulties of solids. I have on my desk right now a bulldog clip:
Given that it is the work of seconds to break it down into its component pieces (and only slightly longer to reassemble it), is this one object or three? Or is it one object when assembled and three objects when disassembled and, of course, two objects when partially disassembled?
What if instead of disassembling it I broke it into 3 pieces with bolt cutters or an angle grinder? Still one object, or does the irreversible nature of the disassembly change this in some way?
Moving onto fluids, is a bottle of Coke an object? It consists of 4 solid pieces (bottle, cap, cap ring and label) and 2 fluid pieces (the Coke and the air). Is this one object? If I cast a spell that allows me to summon one object do I just get one of these components or all of them?
The Pacific Ocean is an object: "A material thing that can be seen and touched". Is it different in some way from a jug of its water?
The problem we have here is akin to Sorites paradox: given a heap of sand from which we remove 1 grain at a time, when does it cease to be a heap? When there is no sand? Surely not because 1 grain is not a heap. 10,000 grains? Surely not because if 10,000 grains is not a heap then 10,001 can't be heap, can it? After all , its just 1 more grain and 1 grain isn't a heap.
So, what's the solution?
Fortunately D&D 5e has a rule for dealing with exactly this sort of ambiguity in its other rules on p.6 of the Player's Handbook: "The DM narrates the results of the adventurers’ actions."