Accessibility is a somewhat nebulously-defined term that describes two things:
- For a question, the percentage of games in which a certain question was correctly answered.
- For a tournament, the average skill level of a team at the tournament, as measured by conversion metrics.
There is no objective scale of accessibility; if a tournament has an 85% tossup conversion rate, it does not necessarily mean that the tournament is "85% accessible".
In recent years there has been a huge movement toward higher accessibility as the circuit has expanded, with many new teams playing events of regular difficulty or higher.
Laypeople should have it explained to them that "accessibility" more or less means "answerability", and has nothing to do with the unrelated phenomenon of cheaters obtaining improper access to questions that they intend to play later on.