There has long been a trend for a crossover between 'sportswear' and 'leisurewear'. (i.e. millions of people who never participate in sports still wear sportswear). It seems that some good marketing tactics, from firms such as Go Outdoors and Millets, are now encouraging a trend for a crossover between hiking gear (such as backpacks) and everyday bags.
It's to be welcomed, of course, because
(a) it's an excellent way to carry loads and far better for one's posture than carrying a heavy bag at one's side ; and
(b) people will no longer automatically suspect that you're a terrorist bomber when you use a backpack on the Tube!
It seems though that the correct term for the smaller type of backpacks that are now being used by many people (rather than the massive rucksacks that are popular with hikers) is, according to the marketers, either a 'daysack' or a 'daypack':
https://www.millets.co.uk/equipment/rucksacks-bags/daysacks/