Note: Many of the entries here will eventually become full-length posts. Some are rough and have not been fully researched. If you have any corrections or would like to add anything, please comment.
Aberrant: deviating from norms. There is also aberrational, meaning or or relating to aberration; and the rare aberrative, meaning tending to deviate from norms.
Absorption: absorbtion is a misspelling.
Accent vs. accentuate: As the traditional distinction goes, accent is literal while accentuate is positive. For example, you might accent the first syllable of a word while accentuating the health benefits of green tea.
Accept vs. except: Accept: to receive willingly. Except: to make an exception.
Acceptance vs. acceptation: Acceptation is on the part of what has been accepted, while acceptation is done by the one accepting. For example, if I offer you twenty cents, the money’s acceptation is not complete until you give your acceptance.
Access (as a verb): Its verb senses are fairly new (dating from the second half of the 20th century) but nonetheless well established, and the development is irreversible. Anyone who still claims access doesn’t work as a verb is fighting for a lost cause.
Accost: Traditionally, it means to approach (someone) in an abrupt or hostile manner without denoting force or violence. But in 21st-century writing, especially newswriting, it often connotes physical force. In such cases, assault or attack works in its place.
Accoutrement: a clothing or equipment accessory. The word is sometimes pronounced in the French manner (ackoo-truh-mahn), but it has been in English for centuries, so there’s nothing wrong with the anglicized spelling.
Accrost (or accrosed, accrossed, acrosed, acrost, acrosst, etc.): used colloquially (especially in sports commentary) for across, but not a dictionary-recognized word.
Accuser: not accusor.
Actualise vs. actualize: American and Canadian English: actualize, actualized, actualizing, actualization, etc. Outside North America: actualise, actualised, actualising, actualisation, etc.
Adaptable vs. adaptive: Adaptable: can adapt. Adaptive: does adapt.
Addenda vs. addendums: Both are widely used. Addendum, though Latin in origin, is a long-established English word, so we can pluralize it in the English manner.
Adduce vs. educe: Adduce: to cite as an example to prove an argument. Educe: to elicit.
Adieu: French for goodbye.
Adjuster: not adjustor. The latter spelling is an accepted variant, but it is used only rarely. The spellings bear no difference in meaning.
Adsorption: the accumulation of molecules of a substance on a surface. Things that are adsorbed are not absorbed because they do not go beyond the surface. Adsorbtion is a misspelling.
Adventuresome vs. adventurous: no difference. Adventurous is much more common.
Advertise vs. advertize: All varieties of English use the -ise form.
Aetiology vs. etiology: etiology in American English; aetiology in British English.
Affluent vs. effluent: Affluent: rich. Effluent: flowing.
Affordability vs. affordableness: Affordability is very much preferred.
Aforesaid: legalese for mentioned earlier. It can sound overformal outside legal contexts.
Aggrandize: to increase the scope of or to increase in power. When something is described as aggrandized, this usually means it has been exaggerated.
Agilely: not agily or agiley.
Agitprop: political propaganda, especially for Communism.
Agog: very eager or impatient.
Agree to vs. agree with: You agree to take a course of action; you agree with a thought or sentiment.
Agreeance: not a dictionary-approved word. Use agreement.
Akimbo: with hands on hips and elbows out.
Alight: (1) to land; (2) burning or lighted.
All-American: American in typical ways. But given America’s diversity, the term is vague and a little antiquated.
All but: almost completely; not quite.
All of a sudden vs. all of the sudden: Some very strict English-speakers disapprove of all of the sudden for arbitrary reasons, but both forms are equally logical.
Alliteration vs. assonance: Alliteration: the repetition of sounds at the beginnings of words and stressed syllables. Assonance: the repetition of vowel sounds.
Alma mater: the school that one has attended. The plural in English is alma maters.
Also-ran: a loser or one that does not have success. It has a hyphen.
Amative, amatory: There is no substantive difference between them, and both usually bear replacement with amorous.
Amends: recompense for insult or injury. It usually takes a plural verb.
Anachronism: something that is out of proper chronological order, especially something that is reminiscent of a bygone time.
Analyses: the undisputed plural of analysis.
Anodyne: soothing or relaxing, or something that soothes or relaxes.
Antarctic: not antartic.
Antediluvian: extremely old. Literally, before the flood.
Anyday: Outside very informal contexts, make it any day. The one-word form is not (yet?) a dictionary-approved word.
Any more vs. anymore: The one-word form is an adverb. Make it two words when it functions adjectivally (e.g., Do you want any more milk?—any more is an adjectival phrase modifying milk).
Any one vs. anyone: If any person would work in its place, use anyone. Use any one if it means (1) any one but not more than one or (2) whatever one of a group.
Apocryphal: of questionable authenticity. It’s usually applied to stories whose veracity cannot be confirmed.
Appellation: fancy word for name.
Approbation: an expression of approval.
Arcane: known by or familiar to only a few.
Arise vs. rise: They are mostly synonymous. Arise appears more often in relation to rising from sleep. Outside sleep-related contexts, it has a more dramatic ring than rise.
As all get-out: to the utmost degree.
As soon vs. assume: as soon is the correct phrase in constructions such as I would just as soon punch him as kiss him.
Assignation vs. assignment: They share the senses the act of assigning and a thing assigned, though assignment is more common in both these senses. Assignation alone bears the sense a secret meeting between lovers.
Astride: with one leg on each side.
At wits’ end: very upset, or at the limits of one’s emotional or mental limitations. It’s commonly spelled at wit’s end, but we say at the end of my wits, not at the end of my wit, so at wits’ end makes more sense.
Au fait: fully informed. French for to the point.
Await vs. wait: Await takes a direct object, meaning it acts directly on something. For example, we say I await you, not I await for you. Wait is the opposite; its object must be indirect. For example, we say I wait for you, not I wait you.
Award vs. reward: Awards are often sought. Rewards are given regardless of whether they are sought.