I am looking for the name of the figure of speech, where two words with similar meaning are used together to convey an idea more emphatically. For example: 'Cease and Desist', 'Null and void', etc.
I tried googling but nothing concrete turns up.
Thanks.
Pleonasm
According to wiki - Pleonasm is the use of more words or parts of words than is necessary for clear expression: examples are black darkness, or burning fire
You have the choice between tautology, accumulatio, hendiadys and pleonasm. See https://en.m.wikipedia.org/wiki/Glossary_of_rhetorical_terms
I don't know of a classical (Latin or Greek) rhetorical figure, but the following might help.
Siamese Twins are, according to Wiki, "a pair or grouping of words used together as an idiomatic expression or collocation, usually conjoined by the words and or or."
Wiki has a whole list of Siamese Twins which involve related and synonymous words, so you might think to call such pairs Siamese Synonyms or Related Twins.
You might like these terms better than pleonasm, since they suggest the presence of a conjunctive element where pleonasm does not.
I believe the term siamese twins is falling out of favour and has been replaced by binomial pairs or binomials which are two words joined by a conjunction (usually and or or) while the word order can be reversed, it is fixed in irreversible binomials . In expressions such as breaking and entering (BrEng), law and order, and cease and desist–the word order is rarely switched; e.g. desist and cease.
Hendiadys, previously mentioned by rogermue, are two related ideas linked by and that needn't be in a fixed order; for example [mine], ‘a letter written in fury and frustration’ or ‘a letter written in frustration and fury’, which means a frustrated furious letter .
