The tilde is the mark "~" placed on top of a symbol to indicate some special property. 
is voiced "
-tilde." The tilde symbol is commonly used to denote an
operator. In informal usage, "tilde" is often instead voiced as "twiddle"
(Derbyshire 2004, p. 45).
1. An operator such as the differential operator 
.
2. The statistical median 
(Kenney and Keeping 1962, p. 211).
The tilde is sometimes used as its own symbol.
1. In asymptotic notation, 
is used to mean that 
.
2. Physicists and astronomers use same notation to mean "
is of the same order of magnitude
as 
."
3. In set theory, 
means that there is an equivalence relation
between 
and 
.
4. In statistics, the tilde is frequently used to mean "has the distribution (of)," for instance, 
means "the stochastic (random) variable 
has the distribution 
(the standard
normal distribution). If 
and 
are stochastic variables then 
means "
has the same distribution as 
.
