the most basic or simplest parts

fundamentals

the part (of a problem) that is the most difficult to solve

most important part

a small, particular fact or item

feature, specific aspect

the nature or most important quality (of something)

heart, significance

the act of establishment (a university, church etc); a strong base of a building, on which it is built

basis for something physical or mental

(used in compounds) full of strong (e.g. good) intentions; what is understood by a word, phrase etc

message, signification

(usually used with a sing verb) the science of motion and force

mechanical details

details

detail

(an example of) evidence that is sufficient to show that something is a fact

evidence, authentication

the first steps or stages (of an art or science)

fundamentals

a special meaning, importance

meaning

an activity, area of study, to which a person gives particular attention; a product for which a business or place is well known

detail

the conditions of an agreement; (used with an 'adj' describing) the kind of language used

conditions, agreement

the quality or state of being true; something that is true; a fact, belief etc accepted as true

reality, validity

basic; hidden but able to be found

fundamental, latent

of, connected, with, necessary for, living; greatest; indispensable

essential