Freemasonry (also known to members are the Craft) means different things to different people and because Freemasonry has no dogma (which is one important reason why it is not a religion) providing a definition which is acceptable to every Freemason is impossible. Here then is presented a, that is one, possible definition. However, there are a couple of definitions which are good starting points:
'Freemasonry tries to make good men better, it cannot make bad men good'
Although this might sound a little trite to some it goes some way towards the central intent of Masonic teaching and practice. Another definition might be:
'A peculiar system of morality, veiled in allegory and illustrated with symbols'.
This sounds rather old fashioned to modern ears so an interpretation of that might be worthwhile. The word 'peculiar' has changed since it was first used to describe Freemasonry. Now-a-days is suggests odd or strange but when we are aware that it originally meant special or unique it makes a little more sense. 'Veiled in allegory' also benefits from further explanation. Allegory is one way of teaching precepts by telling a story and as a technique continues to be used today although has been used of a very long time. There are hundreds of examples to chose from and many are familiar. The story of the Good Samaritan is an example of and allegorical story or parable. 'Illustrated with symbols' is not so straight forward as one might think. Symbols are a form of shorthand. Some Masonic symbols may look familiar and other quite strange. This is known as the 'private language of the Craft' and so non-Masons cannot be expected to know the Masonic meaning(s) of a symbol even when it looks identical to one which they are familar.
Describing Freemasonry 'a peculiar system of morality, veiled in allegory and illustrated with symbols' could be rendered in more modern language as: 'a special and unique system of morality which is communicated by way of allegorical plays which use Masonic symbols as a teaching aid'. [I know which I prefer! - Ed]