Step 1: Understanding the Concept
Natality is the birth rate of a population.
Step 2: Absolute natality
Absolute (maximum) natality is the highest possible number of births under ideal conditions, with no shortage of food or space and no adverse factors.
Step 3: Realised natality
The actual number of births under environmental pressure is the realised (ecological) natality, which is lower.
Final Answer:
Absolute natality is the number of births under ideal conditions.
\[ \boxed{\text{(A) }\text{number of births under ideal conditions.}} \]