side reaction
You set out for one destination, but a tempting shortcut keeps pulling some of your group off the main road. A side reaction is chemistry's version: while the reaction you actually want is running, your reactant is also being siphoned off into some other reaction you did not plan for, quietly draining the species you meant to measure or use.
More precisely, a side reaction is any competing equilibrium that consumes a reactant or product of the main reaction. In analysis the classic culprits are protonation (a ligand grabbing hydrogen ions when the pH is wrong) and complexation (a metal ion being snatched up by some other ligand in solution). Each one ties up a piece of your intended species, so the main reaction does not behave as the simple textbook equation predicts.
Side reactions matter because they can wreck an analysis if ignored — but they can also be controlled. Chemists tame them by adjusting conditions (buffering the pH, adding a masking agent) and account for what remains using a conditional constant, which folds the side reaction into an effective equilibrium constant valid under the actual conditions. The honest point: a side reaction is not a mistake in the chemistry, it is reality being more crowded than a single clean equation, and good method design plans for it.
In an EDTA titration of a metal, EDTA also reacts with hydrogen ions. At low pH this protonation side reaction ties up so much EDTA that the titration fails — which is why analysts buffer such titrations to a high enough pH.
Protonation steals EDTA away from the metal it should bind.
The effect of a side reaction is bundled into a 'conditional' (or 'effective') equilibrium constant, which applies only at the chosen pH and conditions. Change the conditions and the conditional constant changes too, even though the true underlying constant does not.