Definition of "isotope" []

• One of two or more atoms having the same atomic number but different mass numbers. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "In helium that occurs naturally the heavier isotope is more frequent than the lighter one by a factor of about 10 million."
• "The U-235 isotope is the all-important factor making uranium an effective fuel for electric generating plants."
• "As we have said, during the irradiation of heavy elements by neutrons, the neutrons are captured and incorporated in the nucleus, and an isotope is thus formed of the primary substance, and this isotope is radioactive."