What is meant by 'lanthanoid contraction'?

Explanation

### Step-by-Step Solution:

1. **Definition of Lanthanoid Contraction**:
Lanthanoid contraction refers to the gradual decrease in the ionic or atomic radius of the lanthanide series elements as the atomic number increases from lanthanum (La) to lutetium (Lu).

2. **Reason for the Contraction**:
The primary reason for this contraction is the increase in the effective nuclear charge experienced by the outermost electrons. As we move across the lanthanide series, the number of protons in the nucleus increases, which leads to a stronger positive charge.

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