Definition of Clearance, Approval and Instruction
 

Dear All,

Would someone please educate me on these ATC terms: Clearance, Approval and Instruction?

For e.g. is permission to push back from gate a clearance or approval?

Thank you.

At Heathrow it would be an 'approval'. Impossible to 'clear' such a manouver, could be all sorts of conflicting traffic.

As mentioned, perhaps no universal definitions, but in my view:

ATC: "Callsign something Cleared to XXXX via YYY Flight level ZZZ" = Clearance

"Callsign something request deviation left due weather"
ATC: "approved" = approval

ATC: "Callsign something fly heading XXX" = Instruction

Many may disagree regarding headings but just my opinion.

According to the Manual Of ATS Pt.1, a clearance authorises an a/c to proceed under conditions specified by an ATC Unit. Clearances are based solely on known traffic conditions & are required for any flight, or portion of a flight, which is provided with an Air Traffic Control Service or Advisory Service.
Clearances do not constitute authority to violate any regulation established by the Dept. of Transport, CAA, or other appropriate authority for promoting safety of flight operation or for any other purpose. Controllers should not issue clearances which imply permission to breach regulations. This is especially relevant in respect of the low flying rules.
I learnt this "parrot fashion" (like many others) 43 years ago. Amazingly, it still sticks in my mind. Although it may have been reworded by now !

DOC 4444
Air traffic control clearance. Authorization for an aircraft to proceed under conditions specified by an air traffic control unit.
Air traffic control instruction.Directives issued by air traffic control for the purpose of requiring a pilot to take a specific action

Nothing about approved as such.
It is usually referenced to a request.
i.e.
.. to request permission to start engines
.. ATC replies c) START UP APPROVED;

In the aerodrome environment there are a number transmissions which are clearances but which the OP may not consider as a clearance because they don't include a "clear" word. These may include: taxi, RWY crossing and line up. A "clear" word is only used in association with a take off or landing for obvious reasons.

Whilst here I have a query on overseas practice. In YouTube clips and sound bites, I hear controllers clearing an aircraft for takeoff followed by the wind or some other information. In Australia, "cleared for takeoff" must not be followed by anything else except a turn direction with a visual departure.

Are the YouTube clips correct or is it bad practice?

UK based. 'never heard of that rule and I was taught the standard format is "(callsign), runway (nn), cleared for takeoff, [surface] wind (nnn) [degrees] (nn) [knots]".

Words [enclosed] being optional.

I would say what you've heard is correct.