My definition of a synthetic monopoly (henceforth SM) is any small grouping of companies that are capturing the vast majority of economic benefit from a certain business model/product/service, and which collectively have an extremely low likelihood of being disrupted by companies outside of the group (regulation more likely). Further, an SM is likely to maintain or increase its collective market share while simultaneously maintaining or increasing its collective (and attractive) profit margins.
ASML - a monopoly.