Layman’s definition : bitcoin is digital cash and can be used to transmit value between people.
Technical definition: bitcoin is distributed, opensource system that enables transfer of tokens between pseudo-anonymous users.
Explanation of technical definition
distributed means that the processing/storage/running of the software needed for bitcoin to work happens between many computers which are not owned by one group or in one location. Instead the software which powers bitcoin can run on any powerful computer if someone wants to run it. In return for running this software there is a chance that the computer running it may be awarded some bitcoin, as a reward. This means that rather than a single server that can go “down for maintenance” bitcoin is resistant to attack by hackers and cannot be shut down by anyone.
opensource means that the software is not owned by anyone or any company. It is free to use and modify!
transfer of tokens means that people can send tokens to other people by knowing their ‘address’ like an email address
psuedo anonymous means that the ‘address’ does not reveal the identity of the user sending or receiving.