What does "IMSI" stand for in IMSI-catcher?
IMSI-catcher
international mobile subscriber identity-catcher

An international mobile subscriber identity-catcher, or IMSI-catcher, is a telephone eavesdropping device used for intercepting mobile phone traffic and tracking location data of mobile phone users.[1] Essentially a "fake" mobile tower acting between the target mobile phone and the service provider's real towers, it is an example of a man-in-the-middle (MITM) attack. The 3G wireless standard offers some risk mitigation due to mutual authentication required from both the handset and the network.[2] However, sophisticated attacks may be able to downgrade 3G and LTE to non-LTE network services which do not require mutual authentication.[3]