Monster Manual p 6 defines creature types. Humanoids are on p 7.
"Humanoids are the main peoples of the D&D world, both civilized and savage, including humans and a tremendous variety of other species. They have language and culture, few if any innate magical abilities (though most humanoids can learn spellcasting), and a bipedal form. The most common humanoid races are the ones most suitable as player characters: humans, dwarves, elves, and halflings. Almost as numerous but far more savage and brutal, and almost uniformly evil, are the races of goblinoids (golbilins, hobgoblins, and bugbears), orcs, gnolls, lizardfolk, and kobolds. A variety of humanoids appear throughout this book, but the races detailed in the Player's Handbook- with the exception of drow-are dealt with in appendix B. That appendix gives you a number of stat blocks that you can use to make various members of those races."
I guess that passage from the 2014 Monster Manual is the most "official" text describing the Humanoid creature type.
Probably the most useful part is: "Humanoids have language and culture".
For such a brief and vague description, it still has numerous difficulties. I assume the Dragon creature type also has culture, but perhaps they dont, if dragons behave by pure instinct and are unable to learn new things.
Perhaps nature spirits like Giant and Fey lack innate culture, and actually reflect and echo Human cultures.
The term "savage" in contrast to "civilized", along with a reference to "uniformly evil races", seems antiquated by todays ethical sensibilities that strive to avoid echoing the racist tropes from the 1800s and 1900s.
The assumption that Humanoid races are bipedal is typical enough, but seems to beg for exceptions to the rule. Even in the same book, the Merfolk list as a nonbipedal Humanoid race that lacks legs.
The description also mentions vaguely how "few if any" have "innate magical abilities". Perhaps the Elf race is one of these few humanoids who are innately magic. Yet even within the Human race itself, the Sorcerer class describes innately magic humans, typically from a nonhuman source, but possibly native. Likewise, where psionic is a source of magic, it is native to the humans.
Generally, 5.5 requires a deep reassessment about what the Humanoid creature type is, exactly.
The name itself is, these races share characteristics that are "humanlike".
Probably, ethical freewill, along with the capacity of learning, language, and culture to form new behaviors that override instinctive behaviors, are part of a more useful definition for Humanoid.