It is often used in medical terms to describe a condition of weakness or lack of strength, such as:
* Asthenopia: A condition of eye fatigue and weakness.
* Asthenozoospermia: A condition of low sperm motility (weakly moving sperm).
* Asthenia: A general feeling of weakness and fatigue.
So, whenever you encounter "astheno" in a word, it generally indicates a lack of strength or weakness related to that word's meaning.