(Latin: one and a half; normally used as a prefix; from Latin, semis “half” + que “and”)

In chemistry, three atoms or equivalents of the [specified] element or radical are combined with two of another, as in iron sesquioxide, Fe 2O3. Also defined as the proportion of two (of one radical or element) to three (of another).

A reference to a very, very long word.
The use of very, very long words.
Someone who uses very, very long words.
hippopotomonstrosesquipedaliophobia (s) (noun) (no plural)
An abnormal dread of exceptionally long words: Suffering from hippopotomonstrosesquipedaliophobia, Jack is always very distressed with very long terms, so when he writes he aways uses language terminology with no more than 3 syllables!
hyperpolysyllabicsesquipedalianist (s) (noun), hyperpolysyllabicsesquipedalianists (pl)
Anyone who has a fascination for, or who enjoys using, excessively long words: "Mrs. Jones was a hyperpolysyllabicsesquipedalianist who loved to challenge her students to determine the meanings of big words and to determine their etymologies so they could comprehend the multitudes of English words that have come from Latin and Greek sources and are used in medical, scientific, and additional sophisticated applications."
One and a half times as great as another; having the ratio of one and a half to one.
sesquicentenarian (s) (noun), sesquicentenarians (pl)
Someone or something of 150 years of age, or between 150 and 159 or in its sixteenth decade of life or existence: "The local university was celebrating its presence as a sesquicentenarian in the community and the faculty was optimistic that it would continue for another sesquicentenarian despite the fact that they would not be available to see it take place."
sesquicentenary (s) (noun), sesquicentenaries (pl)
A 150 year anniversary.
sesquicentennial (adjective) (not comparable)
Referring to a period of 150 years or pertaining to something that occurs every 150 years; relating to or happening after a period of 150 years.
Having the ratio of two and a half to one, or of five to two.
sesquihora, sesquih. (s) (noun); sesquihorae (pl)
An hour and a half or every one and a half hours: "The term sesquihora, or its abbreviation of sesquih., is used in prescription writing by a druggist with the instruction that certain medicines should be taken at the time that as indicated."
sesquilemma (s) (noun), sesquilemmas (pl)
A predicament in which one choice is less repugnant than another one.
1. The difference between being monolingual and bilingual.
2. A situation in which people know one language and part ("half"?) of another one.
—Coined by William Sherk as seen in his "Introduction" to 500 Years of New Words;
Doubleday Canada Limited, Toronto, Canada; 1983.
Someone who knows one and "a half" languages (or one and "part" of another language).
In music, a vibration interval of the ratio 10:9; a lesser major second.

Remember: You can enjoy seeing many examples of sesquipedalianisms with solutions, starting with Group 1-17, by going to sesquipedalia verba.