Book 1 has 23 definitions, though this number varies by edition. Twenty-three, however, is considered the original number of definitions, although there is some debate about whether the first edition included these definitions and whether they were added later, but before 100 AD. Certainly, some of the definitions existed prior to Euclid.
Even in antiquity, the definitions were frequently discussed and criticized. For example, the first definition attempts to define a point as a thing which has no 'part'. But what is a 'part'? The definition is in terms of what it isn't, and the isn't is not defined. Today it's much easier to define as simply a location in Cartesian coordinate space.
Definition 1:
All editions have essentially the same wording for this definition. If we look at the 888 AD D'Orville 301 manuscript, definition 1 is given by:
A point is that which has no part.
However, there are slight variations. These differences are collated in the table shown below: