An element is a type of matter composed of atoms that all have exactly the same positive charge (or having the same number of proton) on their nuclei.
This definition resolves some sticky situations encountered with older definitions:
1) The definition above doesn't make "element" and "mixture" mutually exclusive classifications of matter. A pure element sample that consists of several isotopes is actually a mixture: the isotopes can be separated by physical means. For example, mass spectrometry physically separates the isotopes of an element. Uranium-235 can be separated from uranium-238 using the fact that gaseous compounds of the two elements will spray from a pinhole into a vacuum at different rates.
2) There isn't any problem with recognising different chemical forms of an element (allotropes) as "elements". For example, oxygen can be found as O, as O2, or as O3, in different regions of the atmosphere. These three allotropes have different chemical behaviours and different chemical formulas, but all are samples of the element oxygen.
Source: General Chemistry Online