Most Definitions Are Not Helpful
To answer the question, ‘What is dowsing?' in a few short words would be like trying to answer the question, ‘What is cooking?' by saying that it means to prepare food.
I bet that wouldn't satisfy any chef at all. It would be true, but it would be so lacking as to be meaningless to anyone who enjoyed cooking.
A better definition of dowsing
So, when a definition of dowsing says something like ‘Dowsing is a means of finding underground water', it manages to say so little to anyone who dowses as to be intensely unsatisfying.
There are so many variations of dowsing, so many uses, that to tease just one out of the many threads is to do an injustice to all the others.
So, I think that it would be easier to adopt Sir Christopher Wren's epitaph written at St Paul's Cathedral, which he designed and built. It reads, ‘If you wish to see my epitaph, look around you.'
So it is with dowsing. If you wish to discover what dowsing is, if you wish to define dowsing, then I suggest that you look around this site.
The definition is not to be found in a simple sentence. It is to be found in the multiple ways it can be applied and in the ways it will change you.
However, if you really, really want a simple definition of dowsing, here it is.
The definition of dowsing
Dowsing is a way of obtaining answers to questions which your rational mind cannot provide. It is like a type of focused intuition.