Definition of ob:

part of speech: noun

A prefix meaning " in the way of"; " against," as in obviate; ob becomes oc before c, as in occasion: of before f, as in offend; o before m, as in omit; and op before p, as in oppose; in bot., ob means " reversed"- thus: " cordate" means heart- shaped, with the attachment at the broad end; " obcordate" means heart- shaped, but with the attachment at the narrow end.

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The art of ascertaining the density, & c., of liquids. ...