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on tiptoe standing or walking on the toes, esp. to proceed quietly or cautiously. [2 definitions]
onto to a position on or on top of. [2 definitions]
onto- existence; being. [2 definitions]
ontogeny the process of biological growth and development of a particular living organism. (Cf. phylogeny.)
ontological of or pertaining to the philosophical study of existence and the nature of reality.
ontological argument an a priori argument for the existence of God that asserts that existence is a perfection and that God is the most perfect being, and therefore that God must exist.
ontology the philosophical study of existence and the nature of reality.
on top at the highest point (of something), or on the uppermost surface (esp. of something that is tall). [3 definitions]
on top of in total control. [3 definitions]
onus an unwanted but necessary task; burden. [2 definitions]
on view in a place visible to the public; on display.
onward toward a point or position ahead, as in space or time. [2 definitions]
on welfare receiving public aid because of poverty, hardship, or need.
-onym name; word.
onyx a kind of banded multi-colored quartz, used as a semiprecious stone. [2 definitions]
oo- ovum; egg.
oocyte a cell in the earliest stage of transformation into a female egg; immature egg.
oodles (informal; sometimes used with a sing. verb) a great quantity.
oogenesis the process whereby female egg cells are produced.
oogonium a reproductive cell in some algae and fungi. [2 definitions]
oolite a variety of limestone made up of small spherical grains. [2 definitions]