From the name, one might expect a suit with an Ace on top and no gaps in the cards below that, but the actual definition is a suit with enough top honors and suit length to be able to run the suit without help from partner.
The number of consecutive top cards required depends on the number of cards held in the suit. Subtract the length of the suit from 13 and divide by 2 and that's how many consecutive top cards are needed.
With a 7-card suit, 13 - 7 = 6 / 2 = 3, so AKQ are needed.
With an 8-card suit, 13 - 8 = 5 / 2 = 2.5, so AKJ are needed.
If a suit is one honor short of being solid, it is referred to as semi-solid.