What is Affix?

Legal Definition
An affix (in modern sense) is a morpheme that is attached to a word stem to form a new word. Affixes may be derivational, like English -ness and pre-, or inflectional, like English plural -s and past tense -ed. They are bound morphemes by definition; prefixes and suffixes may be separable affixes. Affixations, the linguistic process speakers use form different words by adding morphemes (affixes) at the beginning (prefixation), the middle (infixation) or the end (suffixation) of words.
-- Wikipedia
Legal Definition
1. To attach (as in: to affix a notice to a door)

2. To sign (as in: to affix a signature to a document)
Legal Definition
To fix or fasten upon, to attach to, inscribe or impress upon, as a signature, a seal, a trade-mark. Pen. Code N. Y. § 367. To attach, add to, or fasten upon, permanently, as in the case of fixtures annexed to real estate. A thing is deemed to be affixed to land when it is attached to it by the roots, as in the case of trees, vines, or shrubs; or imbedded in it, as in the case of walls; or permanently resting upon it, as in the case of buildings; or permanently attached to what is thus permanent, as by means of cement, plaster, nails, bolts, or screws. Civ. Code Cal. § 660; Civ. Code Mont. 1895, § 1076; McNally v. Connolly, 70 Cal. 3, 11 Pac. 320; Miller v. Wad-dingham (Cal.) 25 Pac. 688, ll L. RA. 510.
-- Black's Law Dictionary