What's the difference between racetrack and holding pattern?
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.



Racetrack procedure

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.


and


  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:


Source.

Holding procedure

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.


Source.

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.