1. [adj]  over an extended time; "a longitudinal study of twins"
2. [adj]  running lengthwise; "a thin longitudinal strip"; "longitudinal measurements of the hull"
3. [adj]  of or relating to lines of longitude; "longitudinal reckoning by the navigator"

LONGITUDINAL is a 12 letter word that starts with L.

1. \Lon`gi*tu"di*nal\, a. [Cf. F. longitudinal.]
   1. Of or pertaining to longitude or length; as, longitudinal
      distance.
   
   2. Extending in length; in the direction of the length;
      running lengthwise, as distinguished from transverse; as,
      the longitudinal diameter of a body. --Cheyne.
   
   

2. \Lon`gi*tu"di*nal\, n.
   A railway sleeper lying parallel with the rail.