Firstly: Defining Dry Ablution
Tayammum linguistically: is seeking out something.
Tayammum technically: worshipping Allah, exalted, through seeking out purifying dust or soil  Sa`id is the word used for this in Islamic law. Al-Fayyumi said: “Sa`id in the language of the Arabs is used to mean different things: the dust upon the face of the earth, the face of the earth itself, and a pathway.” Al-Misbah al-Munir (339/1). Al-Shafi`i said: “The term sa`id is not used except for dust that powders in the air [if made unstable through wind or by striking it].” Al-Umm (66/1). Abu Ishaq al-Zajjaj said: “Sa`id is not itself dust, rather it is the face of the earth, dusted or otherwise.” Ma`ani al-Qur'an wa I`rabih (56/2). to wipe over the face and two hands with.
Secondly: Its Legality
Dry ablution is for the removal of minor hadath with its conditions. Consensus has been quoted on the legality of dry ablution by: al-Nawawi and Shams al-Deen Ibn Qudamah.
Thirdly: Dry Ablution Removing Hadath
Dry ablution removes hadath temporarily until water is available. This is the position of the Hanafis, Zahiris, a group of the Malikis, and a narratin from Ahmad. It is also the choice of Ibn al-Mundhir, Ibn Taymiyyah, al-San`ani, al-Shinqiti, Ibn Baz, and Ibn `Uthaymin.