Words related to is

be (v.)

Old English beon, beom, bion "be, exist, come to be, become, happen," from Proto-Germanic *biju- "I am, I will be." This "b-root" is from PIE root *bheue- "to be, exist, grow," and in addition to the words in English it yielded German present first and second person singular (bin, bist, from Old High German bim "I am," bist "thou art"), Latin perfective tenses of esse (fui "I was," etc.), Old Church Slavonic byti "be," Greek phu- "become," Old Irish bi'u "I am," Lithuanian būti "to be," Russian byt' "to be," etc.

The modern verb to be in its entirety represents the merger of two once-distinct verbs, the "b-root" represented by be and the am/was verb, which was itself a conglomerate. Roger Lass ("Old English") describes the verb as "a collection of semantically related paradigm fragments," while Weekley calls it "an accidental conglomeration from the different Old English dial[ect]s." It is the most irregular verb in Modern English and the most common. Collective in all Germanic languages, it has eight different forms in Modern English:

BE (infinitive, subjunctive, imperative); AM (present 1st person singular); ARE (present 2nd person singular and all plural); IS (present 3rd person singular); WAS (past 1st and 3rd persons singular); WERE (past 2nd person singular, all plural; subjunctive); BEING (progressive & present participle; gerund); BEEN (perfect participle).

The paradigm in Old English was: eom, beo (present 1st person singular); eart, bist (present 2nd person singular); is, bið (present 3rd person singular);  sind, sindon, beoð (present plural in all persons); wæs (past 1st and 3rd person singular); wære (past 2nd person singular); wæron (past plural in all persons); wære (singular subjunctive preterit); wæren (plural subjunctive preterit).

The "b-root" had no past tense in Old English, but often served as future tense of am/was. In 13c. it took the place of the infinitive, participle and imperative forms of am/was. Later its plural forms (we beth, ye ben, they be) became standard in Middle English and it made inroads into the singular (I be, thou beest, he beth), but forms of are claimed this turf in the 1500s and replaced be in the plural. For the origin and evolution of the am/was branches of this tangle, see am and was.

That but this blow Might be the be all, and the end all. ["Macbeth" I.vii.5]

Proto-Indo-European root meaning "to be."

It forms all or part of: absence; absent; am; Bodhisattva; entity; essence; essential; essive; eu-; eucalyptus; Eucharist; Euclidean; Eudora; Eugene; eugenics; eulogy; Eunice; euphemism; euphoria; euthanasia; homoiousian; improve; interest; is; onto-; Parousia; present (adj.) "existing at the time;" present (n.2) "what is offered or given as a gift;" proud; quintessence; represent; satyagraha; sin; sooth; soothe; suttee; swastika; yes.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit asmi, Hittite eimi, Greek esti-, Latin est, Old Church Slavonic jesmi, Lithuanian esmi, Gothic imi, Old English eom, German ist.

entity (n.)

1590s, "being," from Late Latin entitatem (nominative entitas), from ens (genitive entis) "a thing," proposed by Caesar as present participle of esse "be" (see is), to render Greek philosophical term to on "that which is" (from neuter of present participle of einai "to be," from PIE root *es- "to be"). Originally abstract; concrete sense in English is from 1620s.

1847 in representations of African-American vernacular, a contraction of I is (see is), irregular for I am. In Scottish and northern English, a colloquial or dialectal contraction of I shall (1796).

abbreviation of Latin id est, literally "that is;" used in English in the sense of "that is to say." Latin id "it" is from PIE pronominal stem *i- (see yon). For est, see is.

i.e. means that is to say, & introduces another way (more comprehensible to the hearer, driving home the speaker's point better, or otherwise preferable) of putting what has been already said; it does not introduce an example, & when substituted for e.g. in that function ... is a blunder. [Fowler]
id est 
Latin, literally "that is (to say)," from id "that," neuter of is, from PIE pronominal stem *i- (see yon). For est, see is. Usually abbreviated i.e. "to write, or even to say, this in the full instead of in the abbreviated form is now so unusual as to convict one of affectation" [Fowler]. It introduces another way to say something already said, not an example of it (which is e.g.).
isness (n.)
"essence," 1865, in a translation of Hegel, from is + -ness.