I had to agree to Pavlo - it looks like he is able to take a look from the outside on JIRA.
A sub-task is not a sub-task
, it's a sub-issue
.
Probably historical reasons?
I think the idea was to add just one sub-issue type as a child of
a parent issue - just to break down an issue in some partial stages.
It becomes just a sub-task.
But .... fortunately Atlassian did in a consistent way and added the feature
to define the type of a sub-issue. Now Professional/Enterprise customer
are able to define there own whatever the need sub-types.
So, it could be a sub-task to a parent issue, but
it could be also a sub-feature or sub-what-else.
The type of the sub-issue depends on the type the administrator defines.
Check JIRA: "Administration -> Sub-Tasks"
Add New Sub-Task Issue Type
But if you summarize the term, it's:
Add New Sub-Issue Type
If you don't setup multiple sub-issue types, you wouldn't run into this inconsistency.
Even Atlassian uses only one sub-issue type.
I know, a developer prefers to add 10 new features
before changing the name
of an existing feature in all the code, documentation etc. 
But please consider to rename sub-task to sub-issue - it becomes
more consistent and easier to explain to the users.
If you use Jira project to track things that are not tasks, reading "Sub-Tasks" or "Sub-Issues" will be suboptimal and not in the context of the project.
There should be an option to rename headings of an issue, per project. For example, if you want to use a project in Jira to track assets (an old Atlassian blog post was speaking about this).
If the issue represents an asset like a Computer, the sub-task could be a piece of hardware inside that computer (ie. the Hard Drive), if the issue represent a Mobile Phone, a sub-task could be representing the SIM card inside, etc.
Instead of reading "Sub-Tasks" I would like to read "Parts" for example.