Are 'homeothermic' and 'endothermic' synonymous?
Traditionally, the terms poikilotherm and homeotherm were used to refer to organisms that don't regulate their body temperature and that regulate their body temperature, respectively.

However, those terms are not the best ones, regarding their etymology. For instance, poikilos (Greek ποικίλος, "many-coloured") means "diverse", "varied", while homoios (Greek ὅμοιoς) means "similar", "resembling". But some fish, which we define as a poikilotherm, may have a lifetime body temperature variation of 1&#8451; or less, while some mammals like a squirrel, which we define as an homeotherm, may have a daily body temperature variation of more than 20&#8451;. 

Then the terms ectotherms and endotherms appeared, as a new and modern terminology. And they are indeed meant to be that. 

The problem is that a lot of people simply thought that ectotherm was a new name for poikilotherm and that endotherm was a new name for homeotherm, as if they were synonyms. But they are not. Follow me:

(A word of caution before we proceed: unfortunately, there is no organisation that regulates the terms used in the various branches of biology, as IUPAC does for chemistry. Thus, the acceptance of a term depends largely on the consensus among the researchers and the book authors. My answer here describes the nomenclature used by Whithers, Comparative animal physiology, in my opinion the best book on comparative physiology.)

Thermoconforms and Thermoregulators

According to Withers (1992), animals can be classified in two groups, regarding the ability to regulate their body temperature:


Thermoconforms: animals that don't regulate their body temperatures. Their body temperature is pretty much the temperature of their surroundings.
Thermoregulators: animals that actively regulate their body temperature. Their body temperature, therefore, may differ from the temperature of their surroundings.


In Withers' words:


  Most animals are unable to control their body temperature, and Tb passively conforms to their thermal environment; these animals thermoconform [...] Some animals, in contrast, regulate their body temperature often against a substantial thermal gradient between their body and the environment; they thermoregulate. 


By the way, this suffix pattern of _conform and _regulator is common along the book. For instance, Withers talks about osmoconforms and osmoregulators (capable or not of regulating their body osmolarity), and so on.

So, thermoconform and thermoregulator are terms that describe if the animal is capable or not of regulating its body temperature. But those terms do not describe the source of the thermal energy.

Endotherms and Ectotherms

Here comes endotherm and ectotherm. In short:


Endotherm: the main source of heat (thermal energy) is the organism itself.
Ectotherm: the main source of heat (thermal energy) is the environment.


According to Withers:


  The terms ectotherm and endotherm are perhaps the most useful for describing the thermal capabilities of animals because they have a mechanistic basis. An ectotherm is an animal whose thermal balance is predominated by external sources of heat, and its metabolic heat production is insignificant. [...] Endotherms are animals whose thermal balance is predominated by their endogenous metabolic heat production.


Now we can try to combine those terms, and that will led us to the comprehension that none of them is synonymous neither of poikilotherm nor homeotherm: 

All thermoconforms are ectotherms, that's not problematic. But things become complicated when it come to thermoregulators: thermoregulators can be ectotherms or endotherms. 

For instance, most reptiles are ectotherm thermoregulators, but some dinosaurian reptiles, such as the birds (and most probably all other Dinosauria), are endotherm thermoregulators.

This table (ibid) summarizes the possible combinations very well:



Conclusion

It's clear that the terms poikilotherm and homeotherm (or even worse, cold-blooded and warm-blooded) are outdated and inadequate. 

But they should not be simply replaced by the pair ectotherm/endotherm. Ectotherm and endotherm are terms that refer only to the main source of heat. The most adequate terms for describing the ability to regulate the temperature are thermoconform and thermoregulator (which, by the way, are not synonymous with poikilotherm/homeotherm either). 

Source:


Withers, P. (1992). Comparative animal physiology. Forth Worth: Saunders.