Is it ok to call a set of UI elements a control, e.g. can this set of elements be called "precision control"?
Or maybe a control is always an atomic UI element (e.g. a button). If so, the above-mentioned set of 2 buttons and an input should be called "precision controls"
Is it ok to call a set of UI elements a control...
Yes, for sure.
Or maybe a control is always an atomic UI element...
Yes, also.
To be serious I saw both and which definition is better is an opinionated topic.
Given that this is an opinion/option then: for an UI developer you will probably hear 2nd definition and she will maybe call first one CompositeControl, UserControl, Component or whatever her favorite UI framework named a composition of basic controls.
However from designer point of view you will probably call control the logic element to perform an action (or to display some information). Implementation details are not important. It's important to pick one definition and stick with it to do not confuse who will read your documents, don't forget to include this definition in your glossary.
Few examples:
All these said...Call them all with same name. Control. Seriously, it doesn't matter and context clarifies the meaning. As I said logic function is the key and you freely speak about spin control and button control even if spin is made using two buttons. You will be more verbose when required (see previous example) and it will be clear. Too complex, or far from real world, definitions will not be adopted - especially in spoken language - and this will cause misunderstandings (exactly what you try to avoid). For example according to ISO 9241-16:1999 a control is defined as:
graphic, often analogous to physical controls such as dials, radio buttons, which allows a user to directly manipulate data, other objects or their attributes
Note that it's a vague definition and their definition for scroll bar (composed by multiple primitive controls) is (emphasis is mine):
control that allows a user to view objects that extend beyond the size of a displayed related window or list by moving the objects into or out of the available display area; the scroll bar also indicates whether additional information is available
Well, a control can not always be an atomic UI element, if only because input devices and technology (mouse, keyboard, touch screen) are also called controls.
For me the criteria to call a group of controls a control is that the group works as a single unit, delivering some unified input to the system as a result of it's operation. Like in your example.
I would comment this instead of making it an answer, but I can't because I don't yet have 50 reputation. :(
To me, a "control" is an element of the user interface that handles a single input or field. Such an element might have multiple affordances (like the example you provided) but ultimately it represents one thing that the user manipulates in the system. However, I'm sure it varies, meaning slightly different things to different people. On my team, I've heard people use "control" to describe a simple text field, but also a DataGrid...a very complex component with dozens or hundreds of inputs.
Control is also a name used by Microsoft within its .NET framework, so if you hear developers using that term, those controls might be what they're referring to.
