1. a suffix meaning “resembling,” “like,” used in the formation of adjectives and nouns (and often implying an incomplete or imperfect
resemblance to what is indicated by the preceding element)
If the alien has bilateral symmetry, bipedal, with four limbs, and a head; it would be appropriate to use the term humanoid - meaning "human like" in appearance.
As for the political correctness of what to call thinking beings that are not human; I've always liked terms like "being", "sapient", "sentient", "citizen" (if they belong to the same political entity), etc.
As an exercise for the reader...
I'd like to elaborate on @LostinFrance 's answer by using a thought experiment.
Pick 5 of the most intelligent creatures living on Earth. For example, based upon EQ.
Encephalization quotient, or EQ, is a way to measure the brain's
development in an animal (or, more properly, a vertebrate) species. It
could be thought that mere brain size would be indicative of
intelligence, but let's consider this list of 38 animal species,
ordered by brain mass.
Some quick factoids about intelligence
As an average, herbivores (with the notable exception of the elephants) and insectivores stay under
1.0 EQ, while carnivores (especially cetaceans) and omnivores (especially primates) are above; social animals rank
higher than solitary animals (dogs higher than cats, horses and lions
higher than rats). Only a few species approach, reach or exceed 3 EQ,
with both extinct and modern hominids at the very top.
--from the Speculative Evolution link above
So if terrestrial life is an indicator of life in general, we can expect the vast majority of alien intelligence to share these traits too. Carnivorous or Omnivorous social animals. An SF story may include solitary and/or herbivorous animals as exceptions to the standards.
The animals I'll pick:
- Humans - EQ ~ 8.1
- Bonobos - EQ ~ 5.9
- Dolphins - EQ ~ 4.2
- Crows - EQ ~ 4.1
- Cuttlefish - EQ ~ 1.4 about a Husky's (dog) level of intelligence
2/5 of these animals (Humans & Bonobos) belong to the same
- Domain - Eukarya
- Kingdom - Animalia
- Phylum - Chordata
- Subphylum - Verbrata
- Class - Mammalia
- Order - Primates
- Family - Hominidae
With Cetaceans diverging from them at the Order (Order - Cetacea, Family - Delphinidae) level.
Crows are birds which are closely related to mammals and diverging from them at the Class (Class - Aves, Order - Passerifores, Family - Corvidae) level.
Only the cuttlefish are significantly different, diverging at the Phylum level - pretty high in our phylogenetic tree (Phylum - Mollusca, Subphylum - ??, Class - Cephalopoda, Order - Sepiida, Family - Sepiidae).
Nearly everything we think of as intelligent in this world is extremely closely related to each other from a phylogenetic and genealogical perspective. We have the same basic structure (bilateral symmetry based upon a quadrupedal layout), have nearly all the same organs, use the same proteins, enzymes, etc. Our surfaces are mostly similar (with hair and feathers interchangeable).
Even though it's different and really stands out as being different, it too descended from the same basic set of biochemistry as other intelligent life on Earth.
Now imagine an alien life form from a planet which uses a completely different biochemistry. Would it be even more different from all intelligent terrestrial life as cuttlefish are from humans?
Most SF treatment of this problem is similar to Star Trek. Dress the alien up in a strange costume, give the actor some bumps on their forehead, and call them an alien. We go along with the gag for convenience but in reality if scientists discovered aliens like this - we'd have to assume we shared common ancestry and not all that long ago (10s of millions of years at the most).
Enter the Analogs
The universe is not only stranger than we imagine, it is stranger than
we can imagine.
-- J. B. S. Haldane
and so alien life is likely to be.
Because I can't predict what alien life might be like, I'd use something I like to call "analogs". We can assume organisms about our size evolved to life on a planet with the gravity and atmosphere of our planet will have some things similar to us:
- Skinlike structure
- Skeletallike structure
- Organs which perform the same functions ours do
Based upon a study of evolutionary biology, the plan form would be likely to possess bilateral symmetry (human like), radial symmetry, or segmented (worm / insect like) as these allows the same genes to be used repeatedly to make larger body size.
Viva la Difference!
- But the number and type of limbs could be any number from 0 - 100s.
- Their manipulators could be like ours or could be tongues, tails,
antenna, mouth pieces, lips, teeth (analogs), etc.
- I think they'd be more likely to possess a central nervous system but this isn't necessarily so.
- Their sensory organs could detect everything we detect plus other
stuff we can't (taste with their "feet", see into UV/IR, etc.).
- Their primary sense organ could be sound, scent, taste, feeling, etc.
and wouldn't necessarily be vision.
It is possible (but probably uncommon) that aliens from different planets could evolve similar structures from completely different biochemistries. This has happened on Earth in which the eye was developed at least 5 different times by evolutions. Eye structures in most cases are coarsely identical but often have drastically different details (the octopus / squid eye is much better than our mammalian eye - their blood vessels don't interfere with their vision).
So you might find vaguely humanoid intelligent aliens out there. They might be bilaterally symmetric, bipedal quadruped layout, upper limbs devoted to manipulation, lower limbs devoted to locomotion, and possess a central nervous system concentrated in a lobe on the top of the body. That head might possess the bulk of the sensory organs.
But it wouldn't look human to us. The limb's joints would probably swing differently than ours. Their movement would look very strange to us.
In short, aliens would be very alien to us and until/unless we became familiar with them, being around them would make us feel very uneasy (and the reverse would probably be true too).
So what we call that wormy thing?
Ordinary humans, being the lazy and imprecise critters that we are, will probably use references to organisms we are more familiar with when naming alien species with common vernacular. So if an alien creature resembles reptiles to us, we will probably call them reptiloids (regardless of the details of their gross structure). The same goes for other gross physiological appearances. Expect exoskeletal aliens to be called insectoids, flying ones to be called avioids, swimming ones to be called fishoids (that just sounds wrong though), etc.
If writing fiction, you might point out that scientifically speaking the alien creature in question is really nothing like the referenced creature on Earth other than some external factor which lead to the comparison.
I really like the ideas behind White's categorization scheme because it can accommodate most any plan form and alien physiology and any sort of classification would have to do this.
But for appearances, humanoid is appropriate and what to call other thinking entities, being or sentient would be appropriate.