lon·gi·tu·di·nal (lŏn′jĭ-tdn-əl, -tyd-, lôn′-)
a. Of or relating to longitude or length: a longitudinal reckoning by the navigator; made longitudinal measurements of the hull.
b. Concerned with the development of persons or groups over time: a longitudinal study of twins.
2. Placed or running lengthwise: longitudinal stripes.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.