С БОЛЬШИЙ БУКВЫPrepPInvarnonagreeing postmodiffixedWO
(a person, thing, or phenomenon that is) an embodiment of the highest degree of the quality or type described: (spelled) with a capital (letter as specified)
with a capital letter.
Что вам сказать? Это был Момент, Момент с большой буквы. Это была любовь-молния (Рыбаков 1). What can one say? This was the Moment, with a capital M. It was love at first sight (1a).