1. a group of people brought together to give advice and opinions, a group of experts working together on a problem
a government think-tank was formed to deal with immigration
A 'think tank' is an institution, company or organisation who study or monitor a particular subject or area of expertise in order to provide information, solutions or advice. This is a noun used to describe a collection of people who are very knowledgeable about a particular subject and organise themselves around this. In a think-tank, you are paid to research your area of expertise full time in order to contribute to society. You are more likely to hear this used in professional contexts.