The base game is the normal play of a slot: each spin costs a stake and pays according to the paytable. It is everything that happens outside a bonus round.
How much of a game's RTP comes from the base game varies a lot. Some low-volatility games return most of their RTP through base-game wins. Many modern high-volatility games pay a smaller share in the base game and rely on features for the rest, which is why they can go a long time without a meaningful win.
The base game usually includes small random modifiers, such as random wilds or a respin, to vary play. Different reel strips are often used in the base game and in features.
In Great Britain, each base-game spin on an online slot must take at least 2.5 seconds, and the per-spin stake limits apply to it.