  Intelligent systems that aim at mastering language as humans do must deal
with its semantic underspecification, namely, the possibility for a linguistic
signal to convey only part of the information needed for communication to
succeed. Consider the usages of the pronoun they, which can leave the gender
and number of its referent(s) underspecified. Semantic underspecification is
not a bug but a crucial language feature that boosts its storage and processing
efficiency. Indeed, human speakers can quickly and effortlessly integrate
semantically-underspecified linguistic signals with a wide range of
non-linguistic information, e.g., the multimodal context, social or cultural
conventions, and shared knowledge. Standard NLP models have, in principle, no
or limited access to such extra information, while multimodal systems grounding
language into other modalities, such as vision, are naturally equipped to
account for this phenomenon. However, we show that they struggle with it, which
could negatively affect their performance and lead to harmful consequences when
used for applications. In this position paper, we argue that our community
should be aware of semantic underspecification if it aims to develop language
technology that can successfully interact with human users. We discuss some
applications where mastering it is crucial and outline a few directions toward
achieving this goal.
