I found some definitions on the web, but they weren't same.
Some resources mentioned it is sum of the times the aircraft is moving on the ground with its own engines. Some defined it as the time between chocks-on and take-off.
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:
Time from the start of motion of an aircraft, under engine power, until the cessation of motion at the completion of a flight, minus flight time.
Addendum #1 Block time, an industry term in the US, refers to the time the aircraft departs the blocks initiating a flight, until it returns to the blocks at the completion of the flight.
Block time may be dichotomized into flight time and taxi time.
Flight time strict definitions vary, but the general industry term is from the initiation of the take off roll, to the stopping of the aircraft after landing, or the departure of the aircraft from the landing runway, which ever comes first.
Taxi time is the time the aircraft spends in movement or holds on the surface of the airport, prior and subsequent to flight.
While the above is generally practiced industry definitions, there are notable variations, as there is no absolute terminology agreement between the various industry segments in aviation and the regulatory segments. For example, in the US, 14 CFR 1.1 provides a definition of "flight time" which is analogous to "block time" in the industry. However, in Europe, JAR 1.1 defines "flight time" as airborne time. Even that definition varies from the industry common practice of the time from initiation of take off roll until the stopping of the aircraft or clearing the runway.
In the US, block time (aka 14 CFR 1.1 "flight time") is normally logged for pilot experience purposes, and includes taxi time associated with movement of the aircraft to/from the runway and the ramp.
ICAO Annex 1 is slightly different from the US FAA definition.
Since the OP asked for a definition of taxi time, I answered this question with the generally, although not 100% universally, industry defacto terms.
From the Pilot Controller Glossary
TAXI− The movement of an airplane under its own power on the surface of an airport (14 CFR Section 135.100 [Note]). Also, it describes the surface movement of helicopters equipped with wheels.
§135.100 Flight crewmember duties.
(c) For the purposes of this section, critical phases of flight includes all ground operations involving taxi, takeoff and landing, and all other flight operations conducted below 10,000 feet, except cruise flight.
Note: Taxi is defined as “movement of an airplane under its own power on the surface of an airport.”