Dimethicone
In plain terms
A silicone that gives lotions and primers their silky slip and forms a light, breathable film on skin. The common worry, that it suffocates skin or clogs pores, does not hold up.
What the evidence shows
Dimethicone is the silicone behind the silky slip of primers and lotions, and the usual objections to it mostly dissolve on inspection. It does not suffocate skin: the film it forms is flexible and breathable, letting water vapor through while smoothing the surface, which is why it is an approved skin-protectant in barrier and diaper creams. It is not comedogenic for most people, it is too large a molecule to absorb into the body, and despite a persistent online claim, it is not a PFAS, which are a different, fluorine-based class entirely. Safety reviewers in the US, EU, and Canada have all cleared it for cosmetic use, and it is not on California's Proposition 65 list. The real silicone concern lives next door. The volatile cyclic siloxanes, labeled D4, D5, and D6, are the ones flagged for building up in the environment and, for D4, hormone effects, and Europe is restricting them. Dimethicone is the linear cousin, lower on both counts, though it can carry traces of those cyclics from manufacturing and is itself slow to break down in nature. For your skin it is inert and well tested; the open question is environmental, and it is mostly about the cyclics, not this one.
On the label
Listed as dimethicone or polydimethylsiloxane, one of the '-cone' silicones. It is the linear, lower-concern kind; the siloxanes under environmental restriction are the volatile cyclic ones, D4, D5, and D6.