“Secular” is used here as a not-quite-opposite word to “religious.” It is so used in the books I use as my basis for spiritual development,

Beyond the Personality: the beginner’s guide to enlightenment

and

Towards Effortless Activity: the advanced guide to enlightenment.

Here is the particular quote, concerning how experience may be described when using this developmental system:

This book teaches you how to understand reality through experience. The fruits of your experience can be expressed in religious or secular terms, according to your choice. This book is a self-help guide to experiencing the unity that is reality.

(Beyond the Personality, Para 1.1)

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“Spiritual” is used on this site as a not-quite-opposite word to “Material.” Spiritual “things” are not actually things at all, but things like love, aspiration, reverence, intuition – and also hate, denigration and irreverence. Material things, on the other hand, are actually things!

As with Secular/Religious, there is quite a grey area.