Unfamiliar Word
In language, a word is the smallest free form that may be uttered in isolation with semantic or pragmatic content (with literal or practical meaning). This contrasts with a morpheme, which is the smallest unit of meaning but will not necessarily stand on its own. A word may consist of a single morpheme (for example: oh!, rock, red, quick, run, expect), or several (rocks, redness, quickly, running, unexpected), whereas a morpheme may not be able to stand on its own as a word (in the words just mentioned, these are -s, -ness, -ly, -ing, un-, -ed).
A complex word will typically include a root and one or more affixes (rock-s, red-ness, quick-ly, run-ning, un-expect-ed), or more than one root in a compound (black-board, rat-race). Words can be put together to build larger elements of language, such as phrases (a red rock), clauses (I threw a rock), and sentences (He threw a rock too but he missed).
The term word may refer to a spoken word or to a written word, or sometimes to the abstract concept behind either. Spoken words are made up of units of sound called phonemes, and written words of symbols called graphemes, such as the letters of English.
another source also find the definition of word from article which is taken from following address (http://www.sil.org/linguistics/Glossary OfLinguisticTerms/WhatIsA-Word.htm)
A word is a unit which is a constituent at the phrase level and above. It is sometimes identifiable according to such criteria as :
1. being the minimal possible unit in a reply
2. having features such as
a. a regular stress pattern, and
b. phonological changes conditioned by or blocked at word boundaries
3. being the largest unit resistant to insertion of new constituents within its boundaries, or being the smallest constituent that can be moved within a sentence without making the sentence ungrammatical.
A word is sometimes placed, in a hierarchy of grammatical constituents, above the morpheme level and below the phrase level.
Unfamiliar word is word that rarely comes up in sentences, or word that are not really well known. Some writers sometimes use some unfamiliar words on their writing in order as their writing style. Some writers also often give the information surrounding the unfamiliar words, so that the reader can determine the meaning of it. Sometime the unfamiliar word could be determined by context clue and deconstructing the word structure.