Select the word from the list that best fits the definition the time it takes for half of the atoms in an isotope to decay

The right answer is Half-life.

Half-life is the time taken by a substance (molecule, drug or other) to lose half of its pharmacological or physiological activity. Used by extension in the field of radioactivity, the half-life, also called radioactive half-life, is the time at which half of the radioactive nuclei of a source have disintegrated.
The term half-life is often misinterpreted: two half-lives do not correspond to the full life of the product. The half-life is in fact the median of the life of a product, that is to say the duration below which there remains more than 50% of the product, and beyond which there remains less than 50%.

We are not able to see the stars. light prevents our seeing the stars. i hope it kinda : )