Is there any difference between them or they are only synonymous?
What's the difference between racetrack and holding pattern? Are they only synonymous?
When used in the context of holding, racetrack pattern and holding pattern refer most of the time to the same pattern, because a holding pattern is a usually designed as a racetrack. Holding terminology:
Holding (or flying a hold) is a maneuver designed to delay an aircraft already in flight.
A holding pattern is the pattern used to hold, regardless of the design of the pattern.
A racetrack holding pattern is a specific design of holding pattern. From Wikipedia:
A holding pattern for instrument flight rules (IFR) aircraft is usually a racetrack pattern based on a holding fix.
Holding patterns are defined relative to a holding fix and are usually timed because aircraft have to remain within a given protected space around the fix.
However a holding procedure which uses a racetrack pattern and a racetrack procedure which uses a racetrack pattern are two different things aimed at two different purposes.
ICAO makes a specific use of the phrase "racetrack procedure" in Doc 8168 (PANS-OPS):
Racetrack procedure: A procedure designed to enable the aircraft to reduce altitude during the initial approach segment and/or establish the aircraft inbound when the entry into a reversal procedure is not practical.
A racetrack procedure consists of: a) a turn from the inbound track through 180° from overhead the facility or fix on to the outbound track, for 1, 2 or 3 minutes; followed by b) a 180° turn in the same direction to return to the inbound track.
For ICAO, a racetrack procedure is a turn procedure, or course reversal procedure, in the instrument approach procedure, comparable to other procedural turns, and has nothing to do with delaying the aircraft. The procedure turns come in different flavors:
Holding procedure definition from ICAO Doc 8168 (PANS-OPS):
A predetermined maneuver which keeps an aircraft within a specified airspace while awaiting further clearance.
It can be used to delay the aircraft, but also to change its altitude using a holding pattern and stack. Note there is no reference to a specific pattern.
However, when holding, the pattern followed is often a racetrack pattern (more details about this in: How is a standard holding pattern flown?)
Depiction on instrument plates
Holding patterns are depicted using regular lines, they are optional
Procedure turns patterns are depicted using bold lines, meaning they are mandatory. Exceptions are noted "NoPT" with indication of the arriving course.
Adapted from AirNav.com.
Holding patterns are used to delay aircraft when there are other aircraft ahead of them for landing at an airport. The holding pattern looks like a racetrack with either standard right turns or non-standard left turns.
FAA-H-8083-25B Pilots Handbook of Aeronautical Knowledge
Holding pattern. A racetrack pattern, involving two turns and two legs, used to keep an aircraft within a prescribed airspace with respect to a geographic fix. A standard pattern uses right turns; nonstandard patterns use left turns.
Holding patterns are also used on an approach to reverse course and possibly to lose altitude.
FAA-H-8083-16 Instrument Procedures Handbook
If the course reversal is depicted as a racetrack or a teardrop on the chart you must fly it as depicted. Otherwise you have a choice of how to fly it.
AIM 5-4-9 On U.S. Government charts, a barbed arrow indicates the maneuvering side of the outbound course on which the procedure turn is made. Headings are provided for course reversal using the 45 degree type procedure turn. However, the point at which the turn may be commenced and the type and rate of turn is left to the discretion of the pilot (limited by the charted remain within xx NM distance). Some of the options are the 45 degree procedure turn, the racetrack pattern, the teardrop procedure turn, or the 80 degree <--> 260 degree course reversal.
Holding patterns are also used when flying a missed approach. I don’t recall ever seeing a missed approach hold that is not depicted as a racetrack.