Brief, concise.
The word you want me to spell is the word ''concise''.
The opposite of ad infinitum would be concise.
Clear, concise, straight forward, etc.
Here are a couple.His concise speech explained the situation quickly.Tell me in concise detail.
Concise.
His presentation was concise and to the point, leaving the audience with a clear understanding of the key points.
The answer is the word 'terse', which means concise and is found in the wording of the clue: 'concise in let (ters e) tc'.
Verbose is to concise as unscrupulous is to principled. The definition of the word unscrupulous is fairly synonymous with unprincipled.
Concise' means 'appropriately brief or to the point.' Thus, several concise example-sentences of this adjective in action would be as follows: 'The speaker's talk was concise' and 'Being concise is important.'
Plain, simple, concise, uncomplicated.
Prolix seems to be a good one.