U+29E0 Square with Contoured Outline
U+29E0 was added in Unicode version 3.2 in 2002. It belongs to the block
This character is a Math Symbol and is commonly used, that is, in no specific script. The character is also known as D'Alembertian.
The glyph is not a composition. It has no designated width in East Asian texts. In bidirectional text it acts as Other Neutral. When changing direction it is not mirrored. The word that U+29E0 forms with similar adjacent characters prevents a line break inside it.
The Wikipedia has the following information about this codepoint:
In special relativity, electromagnetism and wave theory, the d'Alembert operator (denoted by a box: $\u25fb$), also called the d'Alembertian, wave operator, box operator or sometimes quabla operator (cf. nabla symbol) is the Laplace operator of Minkowski space. The operator is named after French mathematician and physicist Jean le Rond d'Alembert.
In Minkowski space, in standard coordinates (t, x, y, z), it has the form
 $\begin{array}{cc}\u25fb& ={\partial}^{\mu}{\partial}_{\mu}={\eta}^{\mu \nu}{\partial}_{\nu}{\partial}_{\mu}=\frac{1}{{c}^{2}}\frac{{\partial}^{2}}{\partial {t}^{2}}\frac{{\partial}^{2}}{\partial {x}^{2}}\frac{{\partial}^{2}}{\partial {y}^{2}}\frac{{\partial}^{2}}{\partial {z}^{2}}\\ & =\frac{1}{{c}^{2}}\frac{{\partial}^{2}}{\partial {t}^{2}}{\nabla}^{2}=\frac{1}{{c}^{2}}\frac{{\partial}^{2}}{\partial {t}^{2}}\Delta \text{}\text{}.\end{array}$
Here ${\nabla}^{2}:=\Delta $ is the 3dimensional Laplacian and η^{μν} is the inverse Minkowski metric with
 ${\eta}_{00}=1$, ${\eta}_{11}={\eta}_{22}={\eta}_{33}=1$, ${\eta}_{\mu \nu}=0$ for $\mu \ne \nu $.
Note that the μ and ν summation indices range from 0 to 3: see Einstein notation.
(Some authors alternatively use the negative metric signature of (− + + +), with ${\eta}_{00}=1,\phantom{\rule{thickmathspace}{0ex}}{\eta}_{11}={\eta}_{22}={\eta}_{33}=1$.)
Lorentz transformations leave the Minkowski metric invariant, so the d'Alembertian yields a Lorentz scalar. The above coordinate expressions remain valid for the standard coordinates in every inertial frame.
Representations
System  Representation 

Nº  10720 
UTF8  E2 A7 A0 
UTF16  29 E0 
UTF32  00 00 29 E0 
URLQuoted  %E2%A7%A0 
HTML hex reference  ⧠ 
Wrong windows1252 Mojibake  â§ 
alias  D'Alembertian 
Elsewhere
Complete Record
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3.2 (2002)  
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