What does a square mean in a Boolean formula

I am reading the paper Measuring the hardness of SAT instances by Ansótegui, Bonet, Levy and Manyà (Proc. 23rd AAAI Conf. on AI, pp. 222–228, 2008) (PDF) and I would like to know what the square symbol in the following means

Lemma 3 The space satisfies the following three properties:

1. $s(\Gamma \cup \{\Box\})$ = 0
2. For any unsatisfiable formula $\Gamma$, and any partial truth assignment $\phi$, we have $s(\phi(\Gamma))\leq s(\Gamma)$.
3. For any unsatisfiable formula $\Gamma$, if $\Box\notin\Gamma$, then there exists a variable $x$ and an assignment $\phi\colon\{x\}\to\{0,1\}$, such that $s(\phi(\Gamma))\leq s(\Gamma)-1$.

The space of a formula is the minimum measure on formulas that satisfy (1), (2) and (3). In other words, we could define the space as:3

$$s(\Gamma) = \min_{x, \overline{x}\in\Gamma, b\in\{0,1\}} \big\{ \max\{s([x\mapsto b](\Gamma))+1, s([x\mapsto\overline{b}](\Gamma))\}\;\big\}$$ when $\Box\notin\Gamma$, and $s(\Gamma\cup\{\Box\}) = 0$.

3 Note that, since $\Gamma$ is unsatisfiable, it either contains $\Box$ or it contains a variable with both signs.

• It looks like a NULL, that is, a contradiction (that can never be satisfied). It is indeed not well defined in the paper, but I'm not so sure it is a common notation. Commented Sep 26, 2015 at 15:39

Semantically, the empty disjunction is the same as contradiction, but the former is a clause whereas the latter is a formula (or sometimes a constant). The usual symbol for contradiction is $\bot$, read bottom.