This, then, is the definition of khalwah (seclusion) according to the lexicographers.
The authors of the Kuwait Encyclopaedia of Fiqh add that the jurists’ usage of the term does not depart from its linguistic sense.

Scholarly disagreement
Seclusion, understood as one individual being alone with another, is lawful when it occurs between a man and another man, or between a woman and another woman, provided that nothing forbidden takes place—such as seclusion for the purpose of committing a sin.
It is likewise permissible when it is between a man and one of his maḥram relatives, or between a man and his own wife.
Among the cases of lawful seclusion is a man’s withdrawing with an unrelated woman while other people are present and can see them, so that their persons are not hidden from view, and the onlookers can hear their words except for what may be exchanged in a low whisper.

Al-Bukhārī records in his Ṣaḥīḥ, on the authority of Anas ibn Mālik, that “a woman from the Anṣār came to the Prophet ﷺ, and he withdrew with her.” Imām al-Bukhārī entitled the chapter that contains this ḥadith: “Chapter: What is permissible of a man being alone with a woman away from the people.” He then commented: “He is not to be alone with her in such a way that their persons are concealed from the people; rather, they are merely out of earshot if they speak in low tones.”

The scholars agree unanimously that private seclusion with a non-maḥram woman is forbidden.
They differ, however, about the ruling on a man’s being alone with an unrelated woman when there is more than one woman present, or when a group of men withdraws with a single woman.
Al-Nawawī states in al-Majmūʿ that the well-known view in the Shāfiʿī school is that it is permissible for one man to be alone with several women to whom he is not maḥram, since corruption is generally unlikely in that situation; likewise, if two men, or—