We place a tilde over the symbol to indicate that it is relative to the vertical diameter.
British Dictionary definitions for tilde
the diacritical mark (~) placed over a letter to indicate a palatal nasal consonant, as in Spanish señor. This symbol is also used in the International Phonetic Alphabet to represent any nasalized vowel
C19: from Spanish, from Latin titulus title, superscription
character "~" ASCII character 126. Common names are: ITU-T: tilde; squiggle; twiddle; not. Rare: approx; wiggle; swung dash; enyay; INTERCAL: sqiggle (sic). Used as C's prefix bitwise negation operator; and in Unixcsh, GNU Emacs, and elsewhere, to stand for the current user's home directory, or, when prefixed to a login name, for the given user's home directory. The "swung dash" or "approximation" sign is not quite the same as tilde in typeset material but the ASCII tilde serves for both (compare angle brackets). [Has anyone else heard this called "tidal" (as in wave)?] (1996-10-18)