There are a number of distinctions that can be made between the terms moral and ethical or relatedly between morality and ethics.
In some instances, the two terms can be used interchangeably. Charles Larmore for instance clarifies in a footnote in his essay “What is Political Philosophy?”
In contrast to some of his other writings, [Bernard] Williams did not appear in his later political essays such as this one to intend any distinction between the “moral” and the “ethical,” and I take this occasion to point out that I myself will be using “moral” and “ethical” interchangeably.p. 279, n. 2.
But while it is possible to use them interchangeably, there exist important differences. The first is etymological, which is what this post will address. The term “ethical” is derived from the Greek word for custom or habit, ήθος (ethos). Who shall we credit for this derivation? Aristotle. In Book 2 of the Nicomachean Ethics, he says:
Virtue [aretē], then, is of two kinds: intellectual [dianoētikē] and ethical [ēthikē]. Intellectual virtue owes its origin and development mainly to teaching, for which reason its attainment requires experience and time; ethical virtue is a result of habit [ēthos], for which reason it has acquired its name [i.e. ēthike] through a small variation on ‘ēthos’.Nicomachean Ethics, 1103a14–18.
The term “moral” is derived ultimately from the Latin term for, just like Aristotle, established practice or usage, mos. The plural of mos is mores, which is the modern English term for, well, social customs. This derivation arrives via the term moralis which Cicero invents. What? Cicero came up with something Aristotle had already come up with? No. Cicero was translating.
… because it a relates to character, called in Greek ήθος (ēthos), while we usually term that part of philosophy [i.e. ēthikē] ‘the study of character,’ but the suitable course is to add to the Latin language by giving this subject the name moralem [moralis in the nominative].De Fato (On Fate), Section I.
Philosophy was traditionally divided into three parts. Diogenes Laertius (7.39) reports: “Philosophic doctrine, say the Stoics, falls into three parts: one physical [phusikē], another ethical [ēthikē], and the third logical [logikē]. Zeno of Citium was the first to make this division.”
The first line of Immanuel Kant’s Groundwork of the Metaphysics of Morals reads: “Ancient Greek philosophy was divided into three sciences: physics [physik], ethics [ethik] and logic [logik].”
What has to be noted is that at least in this classical (more precisely, pagan) tradition, the one was a translation of the other and to that extent meant the same thing. More in the another post.
Aristotle, Nicomachean Ethics, edited and translated by Roger Crisp, Cambridge Texts in the History of Philosophy (New York: Cambridge University Press, 2000).
Aristotle, Nicomachean Ethics, translated by H. Rackham, Loeb Classical
Library 73 (Cambridge, Mass. and London: Harvard University Press and William Heinemann, 1956).
Charles Larmore, ‘What Is Political Philosophy?’, Journal of Moral Philosophy 10, no. 3 (2013): 276–306, https://doi.org/10.1163/174552412X628896.
Cicero, De Fato, translated by H. Rackham, in Cicero: Volume 3. Loeb Classical Library 349 (Cambridge, Mass.: Harvard University Press and London: William Heinemann, 1942).
Diogenes Laertius, Lives of Eminent Philosophers, translated by R. D. Hicks, vol. 2, Loeb Classical Library 185 (London: William Heinemann and New York: G. P. Putnam’s Sons, 1925).
Immanuel Kant, Groundwork of the Metaphysics of Morals: a German-English Edition, translated by Mary J. Gregor, edited and revised by Jens Timmermann, (Cambridge ; New York: Cambridge University Press, 2011 ).