Acts 15:20

but that we write to them, that they abstain from the pollutions of idols, and from fornication, and from what is strangled, and from blood.

ἀλλὰ ἐπιστεῖλαι αὐτοῖς τοῦ ἀπέχεσθαι τῶν ἀλισγημάτων τῶν εἰδώλων καὶ τῆς πορνείας καὶ τοῦ πνικτοῦ καὶ τοῦ αἵματος.

Greek

What we did

We kept the ASV's wording, in modern English.

Why

ἀλισγήματα, "pollutions,"
occurs only here in the New Testament.
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In the letter itself (15:29) and in 21:25 the same item is named εἰδωλόθυτα, "things sacrificed to idols." "Pollutions of idols" renders the word in this verse as it stands.
πνικτός, "strangled,"
occurs only in this list: here, 15:29 and 21:25.
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It refers to an animal killed without its blood being drained.
πορνεία
is rendered "fornication," as the ASV has it.
The four items
correspond to commands Leviticus 17–18 gives for the stranger (גֵּר) living among Israel as well as for Israel: sacrifices to idols (Lev 17:7–9), sexual immorality (Lev 18), animals not drained of blood (Lev 17:13–15), and blood (Lev 17:10–12).
Manuscripts
A few witnesses, notably Codex Bezae, omit "strangled" and add the Golden Rule.
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The main manuscript tradition, followed here, has all four items.