Flat foot Pes Planus. Pes planus or 'flat foot' is a very common condition in which the foot does not have a normal arch when standing. Weight gain, aging, injuries, or illness may harm the tendons and cause flat foot (or feet) to develop in a person who has already formed arches. Treatment includes 'arch support' or orthotics (over-the-counter or custom made). A 'podiatrist' (foot doctor) may help good luck.
Flatfoot. That is a common response to weight bearing . However, if excessive, it may lead to symptoms. You may first try an otc arch support , but if symptoms persist you should seek professional care.
Flat feet. This is referred to as a flatfoot.
Weak arch. Ligament on plantar surface is weak, flat feet, need arch support, ie. Othotics.
It is flexible. It sounds as though you may have a flexible flatfoot. It should be examined to check the extent of the problem and to see how to stabilize it.