What is the exact definition of Taxi Time?
As always, the definition depends on the context. That is why official documents always included the definitions of important terms used in the text.
For Collaborative Decision Making (CDM) as defined  by EUROCONTROL in the latest CMD manual (2017), taxi time is divided in taxi-out time and taxi-in time.


3.4.2 Definition of Taxi Time
For Airport CDM purposes, taxi time is considered to  be:

For arriving flights: the Actual taXi-In Time (AXIT)  is  the  period  between  the  Actual  Landing  Time   (ALDT) and the Actual
In-Block Time (AIBT)
For  departing  flights:  the  Actual  taXi-Out Time   (AXOT) is the period between the Actual Off-Block Time (AOBT) and the Actual
Take Off Time (ATOT).

For  calculation  purposes  within  the  CDM  Platform, taxi times
will be referred to as estimated taxi-in (EXIT) and estimated taxi-out
(EXOT) as there is no requirement for a scheduled, actual or target
taxi time.

Now that in itself doesn't help if you don't know how the ALDT, AIBT, AOBT and ATOT are defined.
The ALDT is the same as the ATC Actual Time of Arrival, which is the time the aircraft touches down.
The AIBT is the actual time when the parking brakes have been engaged at the parking position. (source)
The AOBT is the time the aircraft pushes back / vacates the parking position. (source)
The ATOT is the same as the ATC Actual Time of Departure, which is the time the aircraft lifts of from the runway.
Now Taxi time defined as above is used in Airport management, Air Traffic Management. It may be different in other areas.
Some observations that I want to make given the above definition:

De-icing on-stand is excluded from the taxi-time. De-icing on a remote de-icing pad is included in the taxi-time.
The take-off run is included in the taxi-time.
The landing run is included in the taxi-time.
Taxiing between stands or to/from maintenance is not defined.