Lakshmana Rekha (Sanskrit: लक्ष्मण रेखा), in some later versions of Ramayana, is a line drawn by Lakshmana around the dwelling he shares with his brother Rama and Rama's wife Sita at Panchavati in the forest of Dandakaranya which now part of the city of Nashik in Maharashtra. The line is meant to protect Sita, while he is away searching for Rama. Ramacharitamanas, the popular North Indian rendering of story of Rama, does not feature the Lakshmana Rekha story in the Aranya Kanda. Neither does the original, the Valmiki Ramayana. However, in Lanka Kanda of the Ramcharitmanas, (35.1) Mandodari rebukes Ravana on his boisterous claims of valour by hinting that his claim of strength and valour is shallow for he could not even cross a small line drawn by Shri Rama's younger brother Lakshmana.
In the story, Rama goes chasing a golden deer (which actually is the Rakshasa Maricha in disguise), and does not return for a long time. When Sita coerces Lakshmana to leave in search of his brother, Lakshmana who cannot bear to see Sita cry in grief, reluctantly decides to go and search for Rama, subject to his condition that Sita not cross the protective line he draws. Anybody other than Rama, Sita and himself attempting to cross the line would be singed by flames erupting from the line. Once Lakshmana leaves in search of Rama, the Rakshasa king Ravana comes in the form of a mendicant and asks Sita for alms. Not expecting a trick, she unsuspectingly crosses the Lakshmana Rekha to provide alms to him and Ravana kidnaps her in his Pushpaka Vimana.
Radhey Shyam Ramayan mentions that the crossing of Lakshamana Rekha by Sita was done absent-mindedly by an anxious Sita only to honour the great Indian tradition of "अतिथि देवो भवः" (Atithi Devo Bhava): the guest is embodiment of a Deva (divine entity). Sita crosses the boundary line only to give alms to Ravana once he insists that alms cannot be accepted across a barrier as having a boundary in between was against the principle of free will of the donor.
Lakshmana Rekha, in modern Indian parlance, refers to a strict convention or a rule, never to be broken. See the American Bright-line rule. It often refers to the ethical limits of an action, traversing which may lead to undesirable consequences. Example of use: