Think of a number. Make it a really big number.
1 divided by a very big number is a fraction that would be very close to zero.
1 over any really big number is close to 0. For example: 1/999
In engineering terms, 1/10 or 1/100 of something is often considered a sufficiently small part as to be negligible in relation to the whole.
Of course, that depends on the accuracy and/or precision required. If you're working on some experiment trying to determine the umpteenth decimal digit of some physical constant, very likely 1% error is not small enough to be negligible.
The standard fractions of a inch are1/64th,1/32nd, 1/16th,1/8th,1/4..depends on how close you want to be to zero...then I guess you could go metric 10th of a mm, 100th of a mm,1000th of a mm since you didn't say a fraction of what...like saying one part per million of whatever...or a gram of salt to a pound of something....all fractions or part of a measurement(1 part of a whole)