What is an orthogonal index?

Question

A table in my area of responsibility of our product has been criticised as having more than one orthogonal index.

What is an orthogonal index?
Why is it bad?
How can the situation be avoided?

--Update--
The back-end database engine isn't necessarily relevant here as our application is database-agnostic. But if it helps, Oracle is one possibility.

The table in question isn't used for financial analysis.

Answer 1

Orthogonal means independent of each other.

No idea why it would be bad. In fact, i usually use secondary indexes (besides the 'id' autoincrement primary key) when there's a common query that has nothing to do with the primary one.