Geoinformatics is the science and the technology which develops and uses information science infrastructure to address the problems of geography, geosciences and related branches of engineering.
Note that gis.stackexchange.com is the main stackexchange site for geoinformatics. I don't think this means that all geoinformatics should be prohibited at earthscience.SE (i.e. that geoinformatics tag is useless here), just that only questions concerning much more "geo" than "informatics" belong here. Those concerning both "geo" and "informatics" about the same, or even focused on "informatics", should be asked on gis.SE, and perhaps even closed as off-topic and migrated away from earthscience.SE. Tag excerpt and wiki should emphasize this.