  The Sensor, Observation, Sample, and Actuator (SOSA) ontology provides a
formal but lightweight general-purpose specification for modeling the
interaction between the entities involved in the acts of observation,
actuation, and sampling. SOSA is the result of rethinking the W3C-XG Semantic
Sensor Network (SSN) ontology based on changes in scope and target audience,
technical developments, and lessons learned over the past years. SOSA also acts
as a replacement of SSN's Stimulus Sensor Observation (SSO) core. It has been
developed by the first joint working group of the Open Geospatial Consortium
(OGC) and the World Wide Web Consortium (W3C) on \emph{Spatial Data on the
Web}. In this work, we motivate the need for SOSA, provide an overview of the
main classes and properties, and briefly discuss its integration with the new
release of the SSN ontology as well as various other alignments to
specifications such as OGC's Observations and Measurements (O\&M),
Dolce-Ultralite (DUL), and other prominent ontologies. We will also touch upon
common modeling problems and application areas related to publishing and
searching observation, sampling, and actuation data on the Web. The SOSA
ontology and standard can be accessed at
\url{https://www.w3.org/TR/vocab-ssn/}.
