In Argentine Tango it is customary to use the word ‘Leader” to identify the role of the dancer that is indicating direction, pace, timing, execution, navigation, floorcraft, vocabulary choices, and/or musical interpretation to a degree.

From time to time, you'll see this this site uses a slightly different structure to differentiate between the person, and the action(s). The reason this was done was to create further clarity in the textual descriptions of what X, Y, and Z was or is doing. Far too often with regards to the dance there is a distinct lack of clarity in the language used to describe something, action, or activity. The usage of this textual shorthand was devised as a way to further clarify. Agree, disagree, it doesn't matter, it's here in the text. Deal with it. 😉 

A ‘Lead’- This is the person in the role.
A ‘lead’ - This is the action. Usually performing the role, while employing or engaging ‘Intention Based Dancing’. 
A ‘L/lead’ - This is both person and act in the role.

The words ‘led’ and ‘leading’ are action words.

From time to time you will also see a similar structure regarding the usage of the word “Leader” and have it depicted as follows:

A ‘Lead/er’ - This is a person in the role of the lead, that is employing resistance, tension, force, and compression to accomplish their goals. Typically this person uses their arm, forearm, and hands to direct the Follower to engage a vocabulary choice in time to the music. 

example usage: “I am an Intention based lead. That person over there is leading their Follower as a Lead/er.