Simple Q&A D&D 3.5 (by RAW) XXVII

[Thurbane]

A 714 continued

The Concentration skill may give some clues as to what constitutes "distracted" (bolding mine):

CONCENTRATION (CON)
Check: You must make a Concentration check whenever you might potentially be distracted (by taking damage, by harsh weather, and so on) while engaged in some action that requires your full attention. Such actions include casting a spell, concentrating on an active spell, directing a spell, using a spell-like ability, or using a skill that would provoke an attack of opportunity. In general, if an action wouldn’t normally provoke an attack of opportunity, you need not make a Concentration check to avoid being distracted.

The sample DC table under Concentration gives some other scenarios: non-damaging spell, vigorous/violent motion, grappled and pinned.