Celsius & centigrade
They mean the same thing: The temperature scale from water’s freezing point (0°) to boiling point (100°), a.k.a., the one that’s not Fahrenheit. Celsius honors the man who invented the scale, Anders Celsius, while centigrade (lowercase) is a more descriptive term (centi = hundred, grade = degree).
In general, just use Celsius. It seems that centigrade is on its way out.
gray & grey
Likewise, these both refer to the same color. Grey is the British spelling (hence Gandalf the Grey), whereas gray is more common in American English.
As a surname, it can go either way. Thomas Gray — an English poet — is best known for his “Elegy Written in a Country Churchyard,” while Earl Grey is known for the tea. When it comes to titles, you have Gray’s Anatomy (the medical reference) but Grey’s Anatomy (the medical drama), as well as Fifty Shades of Grey, all of which refer to surnames.
confidant & confidante
Both of these are a person you confide in. The only real difference is that confidant can be either gender, whereas confidante is specifically female. A bit like actor and actress.
Also, apparently a confidante can be a type of sofa. Weird.
Speaking of which …
sofa & couch
The words have very different linguistic origins. Sofa comes from the Arabic suffah, a long bench for reclining, while couch derives from the Old French coucher, to lie down.
That said, there’s almost zero difference between the two in modern American English, except that sofa perhaps suggests something a bit more formal.
ultimate & penultimate
Ultimate means last — and, by extension, the highest, the most extreme, the greatest.
Penultimate simply means second-to-last. Hence the Monty Python sketch with Michelangelo telling the Pope he’ll paint the Penultimate Supper: the Last-but-one Supper.
Speaking of which …
dinner & supper
For me, dinner and supper are basically the same thing, but a distinction exists. Supper refers to the evening meal, whereas dinner is the main (or biggest) meal of the day, whenever that happens to be. So dinner could be lunch, or dinner could be supper — or theoretically, I suppose, it could be neither one.
selfish & self-centered
Selfish is when you refuse to go visit a friend in crisis because you think your own stuff is more important.
Self-centered is when you don’t know your friend is in crisis.
In my experience, the second one is a lot more dangerous.
monkey & ape & primate
The taxonomy is a bit complicated, and the first two terms are often imprecise in everyday usage, but here’s the gist. Primate is the broadest category, corresponding to an entire biological order within class Mammalia. Primates can be broadly divided into three subgroups: apes, monkeys, and others (such as lemurs and tarsiers). Monkeys are generally smaller than apes, and (unlike apes) generally have tails. Apes are further subdivided into the lesser apes (gibbons) and the great apes (gorillas, chimpanzees, and humans, among others).
If all that’s too complicated for you, here’s the upshot: When someone refers to a chimp or gorilla as a monkey, you can yell at them like the ape you are.
frog & toad
This one gets surprisingly complicated. We have to distinguish between scientific usage and common usage.
In scientific terms, the order Anura comprises all the hopping, ribbiting animals we would normally call “frogs” or “toads.” To a biologist, everything in this order is a frog. There is no precise scientific meaning to the term toad.
In common usage, though, frogs tend to live in wet habitats and have smooth, moist skin and long legs, for leaping; whereas toads tend to live in dry habitats and have dry, bumpy skin and shorter legs, for hopping. And there is some scientific basis for such a distinction. The animals we typically call frogs often belong to the family Ranidae (within the order Anura mentioned above), whereas the animals we call toads often belong to the family Bufonidae. These families, however, are only two of the thirty-three families that make up Anura in all.
Short answer: Frogs are wet and toads are dry, but to a biologist, they’re all frogs.
autumn & fall
Americans use these two terms interchangeably to refer to the season after summer and before winter. Autumn is a bit more precise and sounds a bit more formal, and fall is a bit more descriptive, but otherwise there’s no real difference.
I am told, however, that those Brits across the ocean — while recognizing and occasionally using fall — heavily favor autumn.
speed & velocity
Technically, speed is how fast something is going, whereas velocity encompasses both speed and direction. To get even more precise, velocity is a vector, and speed is the vector’s magnitude. It’s possible to change velocity without changing speed (by changing direction), but a change in speed necessarily means a change in velocity.
In common usage, this distinction is often ignored.
Armageddon & apocalypse
Both words are often used to mean a final battle to determine the fate of mankind, or — even more broadly — any final cataclysm that destroys all or most of civilization. Both have a Christian connotation, though this seems to be fading.
Originally, though, these two terms had separate meanings.
Armageddon comes to us today from a single usage in the Bible, in Revelation 16:16, and it referred not to the final battle per se, but to the place where that battle would occur. (I’ve written about this in more detail elsewhere.)
Apocalypse, by contrast, was a Greek word meaning unveiling or revealing or revelation. Yes, the name of the final book of the Bible simply means apocalypse — a divine revelation, a vision of the future. In fact, the apocalypse is an entire genre of religious writing. Most Christian apocalypses, such as the Apocalypse of Peter, did not make it into the biblical canon.
cantaloupe & honeydew melon
This topic has surfaced before.
There. Glad all that’s finally settled.