1. To take some action or get involved in something in order to change what is happening: The police were called to intervene in the escalating conflict in order to prevent it becoming a big braw or scrimmage.
2. To occur and as a result to stop or delay something from happening: The fire alarm in school intervened or interrupted the teacher's presentation of the topic.
3. To take economic action that is designed to counter a trend in a market: The government intervened in the country's free trade in order to stabilize its currency.