What is the X in X-mas?
inflection in·flec·tion (ĭn-flěk'shən)
An inward bending.
A change in the form of a word to reflect different grammatical functions of the word in a sentence. English has lost most of its inflections. Those that remain are chiefly possessive ('s), as in “the boy's hat”; plural (-s), as in “the three girls”; and past tense (-d or -ed), as in cared. Other inflections are found in pronouns — as in he, him, his — and in irregular words such as think/thought, child/children, and mouse/mice.
in linguistics, the change in the form of a word (in English, usually the addition of endings) to mark such distinctions as tense, person, number, gender, mood, voice, and case. English inflection indicates noun plural (cat, cats), third person singular present tense (I, you, we, they buy; he buys), past tense (we walk, we walked), verbals (called, calling), and comparatives (big, bigger, biggest). Changes within the stem, or main word part, are another type of inflection, as in sing, sang, sung and goose, geese. The paradigm of the Old Icelandic u-stem noun skjoldr ("shield"), for example, includes forms with both internal change and suffixation; the nominative singular form is skjoldr, the genitive singular is skjaldar, and the nominative plural is skildir. Many languages, such as Latin, Spanish, French, and German, have a much more extensive system of inflection. For example, Spanish shows verb distinction for person and number, "I, you, he, they live," vivo, vives, vive, viven ("I live," "you live," "he lives," "they live"). A number of languages, especially non-Indo-European ones, inflect with prefixes and infixes, word parts added before a main part or within the main part. Inflection differs from derivation in that it does not change the part of speech. Derivation uses prefixes and suffixes (e.g., in-, -tion) to form new words (e.g., inform, deletion), which can then take inflections.