A length of time that publishers can make authors wait before they are allowed to make their material open access. For example, if an author publishes an article in a non-open access journal, the publisher may agree to a version of the article being made available green open access but not until, say, 12 months after the journal has published the article first.
The length of embargo periods varies. In general, embargos of between 6 months and 24 months are common.
It is worth noting that RCUK require embargo periods for any research they have funded to be limited. Details are provided in section 3.6 of RCUK Policy on Open Access and Supporting Guidance.
To find out what embargo periods many journals and publishers enforce, follow the guidance on the Check your journal/publisher section of this site.