first meeting; initial gathering
Refers to the very first meeting or assembly of a group, committee, or organization. Often used in formal or organizational contexts.
The first meeting of the new committee is next Monday.
The first meeting between the leaders of the two countries took place.
第一回会合 explicitly means 'the first meeting' in a numbered series, whereas 初会合 simply means the initial meeting without necessarily implying a series.
Compound of 初 (hatsu, 'first') + 会合 (kaigō, 'meeting, assembly').