The definition of  terrorism as defined by thefreedictionary.com is

act of terrorism, terrorism, terrorist act – the calculated use of violence (or the threat of violence) against civilians in order to attain goals that are political or religious or ideological in nature; this is done through intimidation or coercion or instilling fear

Here is a list of animal rights groups who have been found by Animal Wrongs to fit this definition:

• PETA – People for the Ethical Treatment of Animals
• HSUS – Humane Society of the United States
• SSCS – Sea Shepherd Conservation Society
• SHAC – Stop Huntingdon Animal Cruelty