4 parajikas (defeats)
The four parajikas (defeats) are rules entailing expulsion from the monastic Order (Sangha) for life. If a monk or nun breaks any one of the rules he/she is automatically 'defeated' in the holy life and falls from monkhood immediately. He is not allowed to become a monk again in his/her lifetime. Intention is necessary in all these four cases to constitute an offence. The four parajikas for bhikkus are:
1. Sexual intercourse, that is, any voluntary sexual interaction between a bhikkhu or bhikkhuni and a living being.
2. Stealing, that is, the robbery of anything worth more than 1/24 troy ounce of gold (as determined by local law).
3. Intentionally bringing about the death of a human being, even if it is still an embryo — whether by killing the person, arranging for an assassin to kill the person, inciting the person to die, or describing the advantages of death.
4. Deliberately lying to another person that one has attained a superior spiritual state, such as claiming to be an Arahant when one knows one is not, or claiming to have attained one of the jhanas when one knows one hasn't.
(from the Vinaya Pitaka)