Definition of "flank"

Stately colonnades are flank'd with trees.

c. 1728, Christopher Pitt, Epistle to Mr. Spence

[...] Mr. M. N. Rollason points out that on four-track lines on which the fast lines, in the centre, are flanked by the slow lines, and running at speed is permissible on all four, the traveller can enjoy some quite exciting experiences when trains are doing a "neck-and-neck" on adjacent lines.

1944 May and June, “Notes and News: Express Travel on Slow Lines”, in Railway Magazine, page 184

It was a rural railway that served the fertile Fens of Cambridgeshire and Norfolk. But because it flanked public roads and was unfenced (to save costs), it was deemed a tramway and its locomotives had to be fitted with a cowcatcher.

2023 January 11, Richard Foster, “British Rail's weirdest railways...: Wisbech & Upwell Tramway”, in RAIL, number 974, page 46

Cautiously I approached the flank of the cliffs, where they terminated in an abrupt escarpment as though some all powerful hand had broken off a great section of rock and set it upon the surface of the earth.

1918 September–November, Edgar Rice Burroughs, “The Land That Time Forgot”, in The Blue Book Magazine, Chicago, Ill.: Story-press Corp.; republished as chapter VIII, in Hugo Gernsback, editor, Amazing Stories, (please specify |part=I, II, or III), New York, N.Y.: Experimenter Publishing, 1927,

Ahead the flanks of the Pennines gleamed faintly in the moonlight, looking as though they themselves were part of some dry and deserted lunar landscape.

1960 January, G. Freeman Allen, “"Condor"—British Railways' fastest freight train”, in Trains Illustrated, page 48

The hosts also had Paul Robinson to thank for a string of saves, three of them coming against Jerome Thomas, who gave Michel Salgado a torrid time down the left flank.

2011 January 23, Alistair Magowan, “Blackburn 2 - 0 West Brom”, in BBC‎