Slim fit is often confused with skinny fit, but they're not the same. Skinny is cut as tight as the fabric's stretch allows; slim tapers close to the body but keeps a little ease, so it reads as sharp rather than restrictive. On a shirt, slim means a trimmer chest and waist; on jeans, a narrower but not skin-tight leg.
Because slim cuts leave little room to spare, fabric matters — a slim garment with some elastane moves with you, while a rigid one can feel tight the moment you sit down.