that which is allowed as per Islamic law, given in the Qur'an and the Sunna. Commonly refers to that which is lawful and permissible to use or consume within Islam. That which is prohibited is HARAM. See SIN for more discussion. Often, however, there are different interpretations and reports, so that different schools have different lists of halal/harm.
"Everything is permissable" - but not everything is beneficial. "Everything is permissable" - but not everything is constructive. Nobody should seek his own good, but the good of others. (1 Corinthians 10:23-24)