In Merriam-Webster learner's dictionary, "dependence" is defined as "the state of being dependent"
and dependency as "the quality of being dependent; dependence".
According to the meaning, I think "dependence" is more likely to be used in concrete situations and "dependency" in abstract situations. However, in the following examples, I find out the two words are almost interchangeable. Could you please tell me the differences between the two words? Or are there any differences?
Dependence and dependency can both be used in the state/condition of being dependent sense. By definition, all words referencing such "states" are abstract nouns, so I don't see any justification for OP's abstract/concrete distinction in that sense. As you can see from this NGram, dependency has gained ground in recent decades, but both are in common use.
The main usage difference is that dependency can be used in a second sense as a "concrete" noun to mean a person or thing which depends on something/someone else. But note that in the programming context it's not uncommon to see it used to mean a software resource upon which some piece of software depends (i.e. - reversing the need/provide relationship).
In principle, dependence could also be used with that second sense - but as OED points out, all such usages are now either obsolete or archaic.
TL;DR: If you want the easy way out (which looks like the way majority usage is going anyway), you can probably get away with using dependency all the time.
But I must be honest - as a native speaker I'd probably tend to refer to his drug dependency, but his dependence on drugs (maybe because I see one as a problem he has, and the other as something he's doing, I don't know).
There are certainly cases where you can use dependency and cannot use dependence: for example "The UK's overseas dependencies", or "This software releases has dependencies on Unix and Java". So if the dependent things are discrete and countable, it should definitely be "dependency".
I think that "dependency" is usually the thing that you depend on, whereas dependence is the state of depending on it. But there are certainly cases where you could use either interchangeably.
And as others have pointed out, there is potential for ambiguity: if A is dependent on B, then a dependence or dependency (relationship) exists; but referring to either A or B as the dependency demands context.
I am a scientist and I have the (probably wrong) impression that it is better to speak about the dependence of a quantity y on a quantity x than about its dependency. I feel dependency is more for dependency on drugs, for example. Dependency, again as I feel it, implies a privation of freedom for who is dependent, while dependence does not. I can make a histogram of the dependence of the production of coal in the UK on month of the year (meaning as a function of month of the year).
There is a somewhat tricky relationship between these two words. In the Longman dictionary when you search for dependence it starts with:
dependence ... (also dependency) noun[uncountable]
I guess so far the relationship is clear. However this relationship is unilateral. Indeed, dependency has meanings which are not implied by dependence. Again with regard to the Longman dictionary, when you search for "dependency" you see this:
1: [uncountable] dependece
2: [countable] a country that is controlled by another country (e.g. Britain's Caribbean dependencies)
I've been (probably incorrectly) using dependency as a plural for dependence.
If Bob is dependent on oxygen, Bob has a dependence. If he's also dependent on food, he has dependencies.
For what it's worth, my spellcheck doesn't recognize "dependences" as a word, so this "plural" theory seems to be the path of least resistance.
This usage obsoletes the word "dependency", but I'm ok with that.
