  In the coming sixth generation (6G) communication era, to provide seamless
and ubiquitous connections, the space-air-ground-sea integrated network
(SAGSIN) is envisioned to address the challenges of communication coverage in
areas with difficult conditions, such as the forest, desert, and sea.
Considering the fundamental limitations of the SAGSIN including large-scale
scenarios, highly dynamic channels, and limited device capabilities,
traditional communications based on Shannon information theory cannot satisfy
the communication demands. Moreover, bit-level reconstruction is usually
redundant for many human-to-machine or machine-to-machine applications in the
SAGSIN. Therefore, it is imperative to consider high-level communications
towards semantics exchange, called semantic communications. In this survey,
according to the interpretations of the term "semantics", including
"significance", "meaning", and "effectiveness-related information", we review
state-of-the-art works on semantic communications from three perspectives,
which are 1) significance representation and protection, 2) meaning similarity
measurement and meaning enhancement, and 3) ultimate effectiveness and
effectiveness yielding. Sequentially, three types of semantic communication
systems can be correspondingly introduced, namely the significance-oriented,
meaning-oriented, and effectiveness/task-oriented semantic communication
systems. Implementation of the above three types of systems in the SAGSIN
necessitates a new perception-communication-computing-actuation-integrated
paradigm (PCCAIP), where all the available perception, computing, and actuation
techniques jointly facilitates significance-oriented sampling & transmission,
semantic extraction & reconstruction, and task decision. Finally, we point out
some future challenges on semantic communications in the SAGSIN. ...
