Merge pull request #143 from datalad/docs-glossary-update

DOC: make a small addition to the glossary

docs/source/glossary.rst

@@ -5,14 +5,17 @@ Glossary
 
 .. glossary::
 
+   credentials
+       Something used to authenticate to a server, typically a username and a password. Datalad-xnat may prompt you for your credentials or use ones which have been previously saved (in datalad configuration or system keyring, see `datalad docs on credential management <https://docs.datalad.org/en/stable/design/credentials.html>`_). Some XNAT servers allow anonymous access (without checking credentials).
+
    experiment
-       An experiment is an event by which data is acquired. This data can be imaging data, or non-imaging data. It exists within the context of a :term:`project`, but can be registered into multiple :term:`project`\s.
+       In XNAT terms, an experiment is an event by which data is acquired. This data can be imaging data, or non-imaging data. It exists within the context of a :term:`project`, but can be registered into multiple :term:`project`\s. Most experiments will be imaging :term:`session`\s.
 
    project
        A project is used to define a collection of data stored in XNAT. These often correlate directly to an IRB approved study, or a multi-site data acquisition program. Within XNAT, the project is used to define a security structure for data. Users are given certain permissions for data within certain projects -- thus, as a user you may not have permissions for all projects on a given XNAT instance.
 
    session
-       TODO
+       In XNAT, an image session is a specific kind of an :term:`experiment` which contains image data. A session groups together multiple scans, where a scan corresponds to a DICOM series or BIDS data acquisition / run.
 
    subdataset
       A DataLad dataset contained within a different DataLad dataset (the parent or DataLad :term:`superdataset`).