How to add contextual help for tables

I've got a table that contains around 9 columns. Many of the columns contain dates and reference numbers that are used for complicated processes. The column headings are currently expert terms for the data.

The tables are used by experts as well as first-time users. How do I explain to new users what the information in each column is for?

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I've thought about:

  • Adding question mark icons in column headings so that the icons open the tooltips (looks messy)
  • Making the headings as links that open tooltips (but the user might not be clear why the heading is a link, looks like the header should sort the table)
  • Adding a glossary section somewhere (all terms are defined but the user has to hunt for the term she/he wants to learn about. It also gets in the way for expert users that don't need a glossary.)

How could this issue be solved? Any good examples would be welcome.

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