The curve traced out by points P and P′ which lie on lines through a fixed point A where the midpoint M of PP′ is on a fixed line and the absolute value |PM| = the absolute value |P′M| = the absolute value |MO|, where O is the projection of A onto that fixed line.
[Greekstrophos, twisted cord (fromstrephein, to turn; see streb(h)- in the Appendix of Indo-European roots) + –OID.]
(click for a larger image)strophoid
The equation for this strophoid is
x2(a + x)/(a - x).
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 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.