Online Flashcards or Physical flashcards

I recommend physical flashcards as you often need to summarise the concept in fewer words than you would with online; it is also easier to organise into "know" and "don't know piles".
There are studies out there which back-up physical writing of notes etc. to memorise topics - of course, use online for longer definitions etc. if you feel it is convenient, but I'd always recommend you write physical ones and store them in a container to keep them safe.
P.S.) Write flashcards on what you struggle with, not what you already know.