Biometric information is a kind of data that comes from technical processing of personal biological characteristics. Personal biometric characters have the characteristics of invariance and uniqueness, which makes biometric identification identify individuals once and for all. Biometric information processed by a specific technology also has these characteristics, so any attacks on biometric information may have an irreversible impact on individuals. In real life, the information subject suffer loss from illegal commercial collection, freedom limitation, information sale and theft, deep fake and fraud. It's urgent to protect biometric information security, while defining biometric information precisely is the primary prerequisite. Experiences from comparative law show that the biometric information has three essential elements:the reflection of biological characters, the processing by specific technology, and the function of unique recognition. They are the key to identify biometric information. Biometric information can be collected by an image containing personal biometric characteristics, but the image is only a reflection of personal biometric characteristics. The image is not biometric information unless it is processed by specific technology, whether the image is an electronic image or a paper photo. The essence of biometric information is the measurement of personal biometric, which is called "specific technology processing", but the measuring method itself doesn't matter. Whether measurement from face to face on real-person or from image containing personal biometric characteristics, the information is both biometric information. The description of personal biometric characteristic does not constitute biometric information, and personal biometric samples should be distinguished from the personal biometric information. There are many deviations in understanding the definition of biometric information in the legal documents now, and concept confusion, unclear connotation and incomplete extension are the main problems. The definition of biometric information in China should follow three principles in future:revealing the nature of biometric information, reflecting the elements of biometric information, and taking into account the speed of technology progress. It is necessary to define the concept from two aspects:connotation and extension. The functions of connotation are defining essence and composition, so as to provide explanation rooms for possible technological revolution. The functions of extension are listing typical biometric information, so as to provide guide for the current theory and practice. As far as the connotation, "biometric information" can be described as a kind of information which can uniquely identify a natural person by the specific technical processing of his biological characteristics. In terms of extension, the ordinary kinds of biometric information can be listed, at the same time, using the expressing of "including but not limited" to keep the scope of biometric information open.