Wiktionary
n. (context physics English) A line of constant strength in a field
WordNet
n. an imaginary line in a field of force; direction of the line at any point is the direction of the force at that point [syn: line of force]
Wikipedia
A field line is a locus that is defined by a vector field and a starting location within the field. Field lines are useful for visualizing vector fields, which are otherwise hard to depict. Note that, like longitude and latitude lines on a globe, or topographic lines on a topographic map, these lines are not physical lines that are actually present at certain locations; they are merely visualization tools.
Usage examples of "field line".
To the delight of the fans as well as his teammates on the bench, he doubled down the right field line.
He went to wailing on a high fastball and pulled it deep down the right-field line.
What we feel is more the current of the primary magnetic field line we are following, said Palou Koror.
They were nearing the left-field line, having traversed most of the outfield, when Jake broke the silence.
Rob swung and connected for a hard line drive down the left field line, but the ball hooked left and landed in foul territory.
Across the field, the cadets of the Arcturus unit walked out to meet them, stopping beside McKenny at the mid-field line.