prerequisite or requisite
Francium is used rarely and only for research.
The plural of necessity is necessities.
australian
Dihydrogen dioxide - the systematic name after IUPAC rules; a synonym is hydrogen peroxide.
Pure aluminium is expensive.
No.
seldom
It would be not often or infrequently
An unfair comparison can be rewritten as "a discriminatory association". One-sided, lopsided, uneven, unjust, biased
Primarily Protestants use the term 'christening' as a synonym for Baptism. It is rarely heard in proper Catholic circles.
No, let is a verb, or more rarely a noun. The past participle is also let, but it is not used formally as an adjective. The usual adjective is the synonym allowed.
Commonly, thermics may refer to any of several physical topics involving heat. It has also been used as a synonym for thermodynamics, but is only rarely used in this way today.
The term soldier has the synonyms combatant, fighter, or warrior.(The term troop is rarely used in the singular, but "trooper" might fit some uses.)
shed can sometimes be used as a synonym
A synonym for the adjective used is second hand. A synonym for the verb used is employed.
Nonmetal is a frequently used synonym for metalloid.
the answer is that some people think it is a necessity, but it used to be a luxury in the 80's