Full Idea
Millian causation is a version of the Regularity Theory, but with the addition that when claiming that an effect invariably follows from the cause, the cause is not a single factor, but a whole conjunction of necessary and sufficient conditions.
Gist of Idea
Mill's regularity theory of causation is based on an effect preceded by a conjunction of causes
Source
report of John Stuart Mill (System of Logic [1843], p.217) by Stathis Psillos - Causation and Explanation §2.2
Book Reference
Psillos,Stathis: 'Causation and Explanation' [Acumen 2002], p.60
A Reaction
Psillos endorses this as an improvement on Hume. But while we may replicate one event preceding another to get regularity, groups of events are hardly ever identical, so no precise pattern will ever be seen.