A casual dress is the one you put on when you can't be bothered to think about an outfit but still want to look like you made an effort without actually making one. No waistband decisions, no working out if the top goes with the bottom, just one piece and you're dressed. It's the thing you reach for on a weekend errand run, a lunch with a friend, a school pickup where you also need to look vaguely human.
It sits well against a denim jacket or a cropped cardigan when the weather turns, and a pair of trainers or flat sandals underneath keeps the whole thing feeling like you didn't try too hard, which is the entire point. If you've got nothing like this in your wardrobe yet, start with something in jersey or a soft cotton blend rather than anything stiff or structured, because you want to be able to sit on a low chair, bend down for a dropped bag, climb into a car without the fabric pulling or creasing into something you then have to apologise for. A midi length in a simple shape, nothing fitted at the waist, is the easiest one to wear on a day you haven't decided how you feel about your body, which is most days if we're honest. For colour, plain navy, a soft faded black, or a small forgiving print in muted tones will take you further than anything bright or bold, mainly because you won't get bored of it by the third wear and it won't show every mark from a toddler's hands or a coffee spill on the train. Avoid anything too clingy in jersey unless you've tried it sitting down in the changing room, not just standing in front of the mirror, because that's where thin jersey tells on you, clinging in places you didn't plan for. Watch the armholes too, a lot of casual dresses are cut generously through the body but tight through the arm, which is fine until you lift your arm to wave at someone across a car park and feel the seam protest. True to size is rare with this category, so if you're between sizes, go up rather than down, a slightly looser casual dress still looks deliberate, a tight one just looks like a mistake you're stuck in for the day.
Reach for one on any day where you need to look sorted without spending more than two minutes deciding, which, if you're anything like most people, is most days of the week.