Chidush (Hebrew: חִדּוּשׁ; also transliterated as chiddush, hiddush or hidush), sometimes used in its plural form, chidushim (Hebrew: חִדּוּשׁים), is a novel interpretation or approach.   Historically referring to Torah topics,   the term is widely used in rabbinic literature   to describe a form of innovation that is made inside the system of the halakha, as distinguished from shinuy, an innovation outside tradition. 
Chidush comes from the Hebrew root word chadash (Hebrew: חדש), meaning new. The usage of the word in this context originated from the language of Talmudic analysis and argumentation in the Gemara. It passed into Yiddish, where it is at times used informally.
Nachmanides states that it is an "obligation imposed upon us to search through the subjects of the Torah and the precepts and bring to light their hidden contents".
What "powers" Chidushim? MaaYana Shel Torah asks regarding "VaYayLech Moshe" (31:1) - where did he go? and answers that he went into everyone: NichNas Moshe Rabbeinu LeToch ToCho Shel Kol Adam MiYisroel. This, he writes, is the basis of people having/writing ChiDuShim.
Although "any chiddush (novel idea) which a reputable disciple will ever come up with was already given to Moses by Sinai," in one rabbi's understanding of a particular ruling, he wrote: "I have always understood Rabbi Feinstein to be insisting on a balance between innovation and tradition.
Chidushim are the ongoing results of a process and, as a form of Kavod HaTorah, we're required not to forget them. New ways to recall what we learn can be a form of chidush.
There is a difference between issuing a ruling, meaning to "distinguish the case at hand from the precident (sic)... to solve a problem,": footnotes 4 & 5 and an understanding of something. Even in the latter case, he writes "What Rabbi Feinstein means is that one should not be innovative (mechadesh) just to innovate."
One form is called Notarikon.
Another is finding a Gematria.
The above term points to a need for something "old" to be seen in a new light. A multi-volume commentary on Mesillas Yesharim compares and contrasts this to emotional insight, a type of Chidush where "something which is novel emotionally" illuminates the value of an idea one already knew intellectually, and brings "a new internalization".
Among the first post-Geonic writers of chidushim: pp.465–466 are:
By the late sixteenth century, with printing an established technology, hair-splitting distinctions into the treatment of halakic-Talmudic themes became more frequent, with chidush-driven works such as those by:
A counter-intuitive use of the term was the Chasam Sofer's novel interpretation of the phrase Chadash asur min haTorah, ("'new' is forbidden by the Torah"). The phrase as originally used is regarding the laws of keeping kosher, whereas his use was regarding changes being made by the Reform movement in Europe: it was a way of saying no - but using a pun.
Chidush has become assimilated into American English. In its regular use, it means an unusual or innovative idea or point, though the word is also commonly used in an ironic or humorous fashion, so as to imply that the statement in question is nothing new.
Book titles may use the word:
Chidushei father and son:
you need a mesorah for gematria
Sarah Bunin Benor, associate professor of Contemporary Jewish Studies at Hebrew Union College–Jewish Institute of Religion and adjunct associate professor of linguistics at the University of Southern California, will speak on 'Chutzpah to Chidush: A Century of Yiddish-Influenced English in America'