Definition:

Mind maps are diagram used to visually organise information showing hierarchy and relationships among thoughts, information consolidating pieces into whole. This is created around a concept with a image or keyword in the centre to which associated ideas are added as words, images connected through lines. Mind maps can be drawn by hands during meeting, planning. 

Mind maps are considered to be a type of spider diagram. Mind map was popularised by Tony Buzan a British psychologist, author and television personality. Buzan suggests few guidelines to create effective mind maps like starting at centre of blank sheet, using atleast 3 colours, images, symbols to be used, use of keywords, lines connected starting from the central image.

Keep the mind map clear using radial hierarchy or outlines to embrace branches. Mind maps are used as a simplified content management system.

Further Reading:

Book: Mind Maps for Business: Using the ultimate thinking tool to revolutionise how you work by Tony Buzan
https://en.wikipedia.org/wiki/Mind_map