Complex sentence: In specification or discussion of some languages, "default constructor" may additionally refer to any constructor that may be called without arguments, either because it is a nullary constructor or because all of its parameters have default values.
Split into simpler sentences: In specification or discussion of some languages, "default constructor" may additionally refer to any constructor that may be called without arguments. Two cases arise here: either it is a nullary constructor or all of its parameters have default values.