Complex sentence: Einstein originally introduced this term in his pioneering 1917 paper on cosmology, with a very specific motivation: contemporary cosmological thought held the universe to be static, and the additional term was required for constructing static model universes within the framework of general relativity.
Split into simpler sentences: Einstein originally introduced this term in his pioneering 1917 paper on cosmology. He had a very specific motivation for doing so: contemporary cosmological thought held the universe to be static, and the additional term was required for constructing static model universes within the framework of general relativity.