Tayammum: Dry Ablution
Tayammum is the dry substitute for wudu (or, in some situations, for a full ritual bath, ghusl), performed using clean earth or dust rather than water. It exists for two specific circumstances laid out in the Quran (5:6): illness that makes using water harmful, or being unable to find water — which in practice today most commonly means travel, extreme cold where water isn't safely accessible, or a genuine, verified water shortage rather than simple inconvenience.
The method is short by design. Strike both palms lightly on clean earth, dust, sand, or a stone surface, then wipe the face once with both hands, followed by wiping each arm — hand to wrist, or in some interpretations further up the forearm, depending on the school of thought — with the dust that remains on the palms. Unlike wudu's multiple washes per limb, tayammum is performed once through, without the three-times repetition used in the water-based ablution.
A key ruling that surprises people learning about tayammum for the first time: it does not carry over between prayers the way wudu does. Each prayer performed with tayammum in place of wudu generally requires a fresh tayammum, because the underlying reasoning is that it's a temporary accommodation tied to the specific inability to access water at that moment, not a permanent substitute equal in standing to wudu itself — though there is some difference of opinion among scholars on exactly how this applies in every case. As soon as water becomes available and there's no longer a valid excuse (the illness resolves, water is found), tayammum is no longer valid for future prayers and wudu with water becomes required again — tayammum does not need to be "undone," it simply stops being the applicable method once its underlying reason is gone.
For illness specifically, tayammum applies not only when using water would be actively harmful (worsening a wound or condition) but, according to many scholars, also when a person has been medically advised that water contact with a particular area would delay healing — a case that comes up often with post-surgical dressings, IV lines, or skin conditions, and is one of the more common real-world reasons tayammum gets used outside of travel.